K1o0n's blog

Codeforces Round #957 (Div. 3)
By K1o0n, 4 months ago, translation, In English

Hello, Codeforces! We are pleased to invite you to Codeforces Round 957 (Div. 3), which will be held on Jul/11/2024 17:35 (Moscow time).

The round will be hosted by rules of educational rounds (extended ICPC). You will be offered 7 problems, which I with Noobish_Monk, ErnKor, Kmes and ArSarapkin have prepared for you.

The rules of the third division are:

  • The penalty for a wrong submission is 10 minutes.

  • 12-hour phase of open hacks after the end of the round (hacks do not give additional points)

  • after the end of the open hacking phase, all solutions will be tested on the updated set of tests, and the ratings recalculated.

Reminding you, that only trusted participants of the third division will be included in the official standings table. This is a forced measure for combating unsporting behavior. To qualify as a trusted participant of the third division, you must:

  • take part in at least five rated rounds (and solve at least one problem in each of them),

  • do not have a point of 1900 or higher in the rating.

Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600 (or you are a newcomer/unrated), then the round will be rated for you.

We would like to thank everybody, who helped in preparing this round:

Good luck!

UPD1 After the end of the round, Shayan will broadcast the solutions, a recording will also be available, but we will also prepare a textual editorial.

UPD2 Video Discussion by Shayan

UPD3 Video of solving the round + Editorial by the "winner" of this round, neal

UPD4 Editorial.

UPD5 Congratulations for the winners:

Official:

  1. siiiuuuuuuu

  2. shil_pelmen

  3. xiojoy

  4. _Separation

  5. son_and_moon

Official + Unofficial:

  1. tourist

  2. neal

  3. Dominater069

  4. abc864197532

  5. hitonanode

  • Vote: I like it
  • +187
  • Vote: I do not like it

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4 months ago, # |
  Vote: I like it +15 Vote: I do not like it

cool

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4 months ago, # |
  Vote: I like it -16 Vote: I do not like it

my goal to reach +1700 after this round

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Hoping for the best Div.3 Round! <3

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

__ combating unsporting behavior. To qualify as a trusted participant of the fourth division, you must: __ take part in at least five rated rounds (and solve at least one problem in each of them), __ do not have a point of 1900 or higher in the rating. __ Regardless of whether you are a trusted participant of the fourth division or not, if your rating is less than 1600 (or you are a newcomer/unrated), then the round will be rated for you.

why is it fourth division

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4 months ago, # |
  Vote: I like it +10 Vote: I do not like it

Finally I can write this: my first unrated div3 :D

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    4 months ago, # ^ |
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    so people rated above 1600 wont be placed on leaderboard ?

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      4 months ago, # ^ |
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      They wont be placed on official leaderboard

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    4 months ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    How I used to think , it will be div 3 which will carry me to expert, but the div 2 hard carried man

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4 months ago, # |
  Vote: I like it +8 Vote: I do not like it

As a tester of this round, I will say that the tasks were interesting

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Hoping to get a good delta positive after this round! I wish everyone the same!

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4 months ago, # |
  Vote: I like it +14 Vote: I do not like it

Just curious: How do you guys propose a Div3 contest?

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4 months ago, # |
  Vote: I like it +2 Vote: I do not like it

Hoping we all get positive delta :)

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Hoping we all learn something regardless of delta :). (No offense, but if someone gets positive delta, someone else won't get positive delta)... I like being realistic.

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4 months ago, # |
  Vote: I like it +17 Vote: I do not like it

As a tester i confirm that problems are indeed cool and worth solving.

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4 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Harsh_kunwar orz

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it

finally a div3 after a long time.

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4 months ago, # |
← Rev. 2   Vote: I like it +37 Vote: I do not like it

Can you tell me how to propose div3 contests? Vladosiya, Gornak40 ?

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4 months ago, # |
  Vote: I like it +13 Vote: I do not like it
Vladosiya for deleting the lore in almost all of the problems...

Thank you.

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4 months ago, # |
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My goal is to solve atleast 5 questions. Let's hope for the best.

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4 months ago, # |
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f(n)=(n−1)⋅(f(n−2)+f(n−1))

can we solve this eqn using matrix expo???

if yes then plzz give me the matrix...

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4 months ago, # |
  Vote: I like it +11 Vote: I do not like it

I like the monk's organization :D

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4 months ago, # |
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Hoping a fresh and exciting contest

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4 months ago, # |
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I want to go specialist, Good luck for me

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4 months ago, # |
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chat do i rise back up to pupil

UPD: I think I do

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4 months ago, # |
  Vote: I like it -14 Vote: I do not like it

Shayan streams pretty much in recent month for problems' solution. I appreciate him a lot.

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4 months ago, # |
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Hope for rating++;

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4 months ago, # |
  Vote: I like it +17 Vote: I do not like it

As a tester, I can guarantee that the round will be interesting!

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

potato

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4 months ago, # |
← Rev. 3   Vote: I like it +9 Vote: I do not like it

I want to solve 3 questions in this contest :)

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it

How to be tester? Is this random?

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    4 months ago, # ^ |
      Vote: I like it +2 Vote: I do not like it

    No, it's not random. Most testers are friends of setters or coordinators. Yes, there are some "random" testers if you want to say it like that, but in a way that they are friends of friends.

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4 months ago, # |
← Rev. 2   Vote: I like it +2 Vote: I do not like it

As a problemsetter, good luck, guys!

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4 months ago, # |
  Vote: I like it +10 Vote: I do not like it

As a tester of this round, I can say that this round will probably lower your rating, but the tasks are worth solving <3

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4 months ago, # |
  Vote: I like it -12 Vote: I do not like it

Can anyone tell me how much problem should I solve by looking at my profile and of what rating in order to get to pupil?

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4 months ago, # |
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a general doubt, penalty gets applied after correct submisson or doen't depend on correct submission?

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    4 months ago, # ^ |
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    Penalty gets applied after correct submission.

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      4 months ago, # ^ |
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      what if I submitted two correct submissions to a problem what is there any penalty on this??

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it

time to lock in

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4 months ago, # |
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good luck guys :)

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4 months ago, # |
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Great! Looking to get a fat positive (will get the opposite).

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4 months ago, # |
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I hope I'll be +1300 after this round! Good luck to everyone!

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4 months ago, # |
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good lucky for all !

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4 months ago, # |
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i wish i'll reach back to 1300

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it

Looking forward to getting my first color today!

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4 months ago, # |
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will trusted participant be rated ??

or just be included in the official standing ?

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    4 months ago, # ^ |
    ← Rev. 4   Vote: I like it +6 Vote: I do not like it

    Regardless of whether you are a trusted participant of the third division or not, if your rating is less than 1600 (or you are a newcomer/unrated), then the round will be rated for you.

    If the rating range is 1600-1900(didn't cross 1900 at any time), then you would only be in the official standings.

    Btw, Congrats!! on reaching Expert

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4 months ago, # |
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I hope to go forward and wish for this to be the best Div3. Odin is with us!!!

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4 months ago, # |
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Looking forward to getting a good positive delta this round

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4 months ago, # |
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My goal above 1200 in this round

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Hoping For a +ve delta this round and learn smtng neww....

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4 months ago, # |
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interesting problems... i solved 3 in 40 minutes :)

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4 months ago, # |
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that went a bit better than last div2 (emphasis on a bit because i somehow crashed my ide 4 times which lost me about 4-6 minutes from E-G)

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4 months ago, # |
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NICE contest, at least for me its a good div 3, i solved 3. some late submission because of cloudflare but it was good overall.

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4 months ago, # |
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got stuck on F , ughhhhhhhhhhhhhhh.

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4 months ago, # |
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Overall very nice problems and good contest. My first time solving E and hopefully I get a good delta :)

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    is it just brute force ? 1e4*1e4 *100 it should not pass obviously

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      4 months ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      here's an oberservation: n*a-b can only be at most 7 digits which means b can only have at most 7 values for each a

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      4 months ago, # ^ |
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      well nope, brute force is TLE, its basically going over all values of A (10^4 iterations) and the it is guaranteed that the length of a valid string after removing b bits from it will never exceed a length of 7 (cuz the constraints come into play) so for every A , you run 7 inner loops and then obviously you can find b in constant time using a formula.

      270014350

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        4 months ago, # ^ |
        ← Rev. 2   Vote: I like it +1 Vote: I do not like it

        you can also solve this with math i believe — start with an empty string (denoted as s), add the digits of n one-by-one to it, then solve the system of equations:

        (int)s = n * a - b

        numdigits(n) * a - b = numdigits(s)

        270034846

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          4 months ago, # ^ |
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          ohh, thanks for this :) . During the contest, i was constantly trying to think in this direction cuz this is much more efficient than what i wrote but yeah my brain just didnt work. Thanks for the soln, appreciated.

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          4 months ago, # ^ |
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          lol I did this but it messes up the case n=1 where solving for a has “infinite”(a up to 10,000 from constraint) solutions. I basically cried after the contest…

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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      how is it 1e4*1e4*100 in brute force....??? beacuse after these 3 loops for(n>9) we have to convert it into string then add it to string for a times and pop back it for b times ....to get the final string that we convert into integer of long long??

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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      are you thinking to pre calculate a-b or a-b/2 or a-b/3 respectively for single digit ,double digit and triple digit......?????

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

C is the easiest but it was the hardest for me to write

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4 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

ignore this was my worst performance soo far

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    4 months ago, # ^ |
      Vote: I like it +2 Vote: I do not like it

    Probably Cheating

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    idk, D is just case handling, and E is running a loop on A and computing B accordingly. As a pupil , D was simple but E took some time

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      4 months ago, # ^ |
      ← Rev. 2   Vote: I like it +1 Vote: I do not like it

      may be this time my brain stopped working possible after getting 3 penalties on A

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        4 months ago, # ^ |
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        3 pens on A is ROUGH tbh :)

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        4 months ago, # ^ |
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        us bro us... similar thing happened with me in D ..after which i just gave up as my brain froze.....I thought E and F would be hard to make it even though i had 50 min left......It was after the contest when I solved E and F ...and since then I'm cursing myself why didn't I moved to E and F

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    4 months ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    D was only matter of implementation, Russian cper have quite different sense of indexing something.

    like if there is single 'W' between 'L' — "LWL". most people probably would think that simply jump over 'W' without swim, no it wasn't you need to jump first and then swim 1 kek.

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      4 months ago, # ^ |
      ← Rev. 3   Vote: I like it 0 Vote: I do not like it

      wasnt it stated you start at n = 0 (ok to be fair it isnt stated the rest of the inputs are [1..n]) (but it doesnt matter that much)

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    4 months ago, # ^ |
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    most prolly mass cheating in D

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    4 months ago, # ^ |
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    How did you get TLE,WA on A despite being a 1550?

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4 months ago, # |
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How to solve F?

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    4 months ago, # ^ |
      Vote: I like it +7 Vote: I do not like it

    set trivializes, only add if its a divisor of x, max 36 elements in the set so it wont tle

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      4 months ago, # ^ |
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      How did I not see this..

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      4 months ago, # ^ |
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      wow so i was in the right direction just less time ughhh. Used sets but couldnt write correct code fack

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      4 months ago, # ^ |
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      How do you come up with 36?

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      4 months ago, # ^ |
        Vote: I like it +4 Vote: I do not like it

      bruh 1e5 has 36 factors

      but 90000 has 74 factors lol

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        4 months ago, # ^ |
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        i dont think that matters, simply going over every factor that can be created does run well in time. 270114245 . And now i feel sed cuz i could have gotten this during contest :(. I wrote the same thing at night just missed the s.insert(1) line during reset of the set

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      4 months ago, # ^ |
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      Actually number of divisors of $$$M$$$ is bounded by $$$2\sqrt M$$$ (think of the way we find divisors) In this case the overall time complexity would be $$$O(N\sqrt N)$$$ for $$$N \sim 10^5$$$ which's fast enough

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        4 months ago, # ^ |
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        ig for all practical purposes N^1/3 can be used as an upper bound for number of divisors

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4 months ago, # |
  Vote: I like it -16 Vote: I do not like it

I literally cannot believe on how ChatGPT unable to solve stupid implementation problem like D.

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    4 months ago, # ^ |
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    it's case handling and reasoning and greedy thinking I think it's very hard for an ai

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    4 months ago, # ^ |
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    why would you want chat gpt to solve for you?

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4 months ago, # |
  Vote: I like it -8 Vote: I do not like it

Hi All, can anyone help me understand why I am getting memory limit exceeded on problem D? this is my submission, I followed a recursive approach to explore possible options of jumping and swimming, I am not using an extra data structure, thank you in advance for your help!

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    4 months ago, # ^ |
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    try passing the string by reference

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      4 months ago, # ^ |
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      Just tried it here, I got time limit exceeded this time; thank you for your help!, I will think on another approach.

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        4 months ago, # ^ |
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        Recursion might not pass with the constraints. I think solving the problem greedily is definitely more straightforward and easier to implement.

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4 months ago, # |
← Rev. 2   Vote: I like it +11 Vote: I do not like it

Was it only for me ,or it was really hard to decode the problems?

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    4 months ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    oh yeah. very weird descriptions. probably their native language is not English.

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      4 months ago, # ^ |
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      None of codeforces, codechef, atcoder speak English natively. Poor preparation is the only reason behind bad statements.

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4 months ago, # |
  Vote: I like it +2 Vote: I do not like it

Another round dusted by Indian cheaters.

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4 months ago, # |
  Vote: I like it +5 Vote: I do not like it

Me staring at E for 1hr +

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4 months ago, # |
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My brain got blown away after reading E was like another level for me.DIV3's have become harder now

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4 months ago, # |
  Vote: I like it +72 Vote: I do not like it

Sharing my video of winning* the contest: https://www.youtube.com/watch?v=9Vv2ZukG1CM&t=2476s

(*tourist doesn't count)

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4 months ago, # |
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Missed E, and actually missed the first time that I could clear Div3.

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    4 months ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    That case n = 1 lol XD

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      4 months ago, # ^ |
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      Yeah, I've just checked it. My solution deserves to get WA because of my stupidity while not considering all the cases =((

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        4 months ago, # ^ |
          Vote: I like it +2 Vote: I do not like it

        btw big fan, i am trying to solve your previous problems when you were pupil/specialist for practice :)

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    4 months ago, # ^ |
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    its okay bro. I got E in like 1hr20 min but i got 2 WA bcoz of fact that string cant be empty. I was so tensed for next 40 minutes and actually we have just 100 different cases so i even tried to find where i was wrong until i read string cant be empty. By that time it was already 2hr10min and penalty was too harsh + no time for F

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    4 months ago, # ^ |
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    your rating does not get reevaluated on this right?

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    4 months ago, # ^ |
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    Can you please review this and help me where i am going wrong. Thanks in advance!

    Code
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4 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Ahh, D was so straightforward but I spent like an hour debugging just to get AC in 3 minutes after reimplementing my idea.

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4 months ago, # |
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What is the thing you have to notice to solve E?

The only way I would be able to solve (I didn't do this) would be to precompute all of n from 2 to 100 and then have a python program just return those precomputed solutions...

How do we solve for B given we check for values of A... Maybe I wasn't on the right track at all for optimal solution though...

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    4 months ago, # ^ |
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    Notice that B is also bounded by 10000, so the length of good results is bounded by len(10000*100-1) = 6. So you can simply iterate all strings of length 1-6 for each A

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4 months ago, # |
  Vote: I like it +2 Vote: I do not like it

Easier than recent div4 round!

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    4 months ago, # ^ |
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    true only was able to solve 4 in that. though i have become better :)

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4 months ago, # |
← Rev. 4   Vote: I like it +57 Vote: I do not like it

The problems were great, but the confusing statements ruined it for me, I really think more attention should be given to making the statement clear and understandable.

For example in problem D it was not clear that k was for the entire problem, when I read it I thought he could not swim a distance more than k in a row which led to me writing a completely different solution and wasted a lot of time on this.

I spent more time understanding the problems than solving them.

I am not saying the contest is bad, but I felt this should be addressed.

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    4 months ago, # ^ |
    ← Rev. 2   Vote: I like it +3 Vote: I do not like it

    Same, initially i also thought it should be k consecutive rows. Luckily i decided to read samples where answer is NO and one of the test case explained where K need not be in a row

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    4 months ago, # ^ |
    ← Rev. 2   Vote: I like it +4 Vote: I do not like it

    I think you just misread they wrote that k is for the entire swim from 1...n

    The river is very cold. Therefore, in total (that is, throughout the entire swim from 0 to n+1 )

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    4 months ago, # ^ |
    ← Rev. 2   Vote: I like it +1 Vote: I do not like it

    Worst Problem was D . First time i solved so poor in DIV3 .

    The first line of each test case contains three numbers n,m,k(0≤k≤2⋅105,1≤n≤2⋅105,1≤m≤10) — the length of the river, the distance ErnKor can jump, and the number of meters ErnKor can swim without freezing.

    I read n then m then k and hence exchanged k's constraint with m's constraint . Stucked in the whole contest for more than 1 hour while solution is correct.

    https://mirror.codeforces.com/contest/1992/submission/270048258

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    4 months ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    same

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    4 months ago, # ^ |
      Vote: I like it +28 Vote: I do not like it

    i mentioned this issue in testing, but unfortunately they did not take it into account

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Actually I also choked because of this , but I also confused about the fact that how to solve this if the task is to compute if swimmer can swim the river if he can continously swim for only k units
    . Please explain you solution for this above wrongly interprated question . Thank you

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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      If you are asking about my solution if you could swim for at most k meters continuously, Here is what I tried to do (Not sure if it's correct or not)

      I used a prefix sum array to count the number of crocodiles, there is a crocodile between l and r if pre[r] - pre[l - 1] > 0

      I used a variable to store where the last log was, If there is water at the current index I check if the distance between it and the last log + m is less than or equal k and that there is no crocodile between log + m and the current index then the current index is reachable.

      The rest is similar to the solution of problem D

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        4 months ago, # ^ |
          Vote: I like it +3 Vote: I do not like it

        Ya make sense , It will pass thank's bro

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4 months ago, # |
  Vote: I like it +2 Vote: I do not like it

There are so many similar solutions for problem D which have been taken from youtube. They all have just changed variable names while initialising a variable name with m-1 and incrementing a variable if there is water and using some other variables and at last they are checking if variable value is not zero then print yes else no. Loads and Loads of solution in the same format, I had a brainrot seeing 10k+ submissions for D itself.

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    4 months ago, # ^ |
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    same I was not able to solve it and D got 10k submission is just blatant cheating D doesn't get 10k+ submission on div 4 how can it get here

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4 months ago, # |
  Vote: I like it +8 Vote: I do not like it

I might have gotten E in contest time itself but cloudflare had other plans.

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4 months ago, # |
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Am I really dumb as from submission its looking like D is easy but I was not able to solve it

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    D was ezz, not 10K easy tho :)

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      4 months ago, # ^ |
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      Yeah 5k submission I can understand as I am dumb but 10k is just crazy I am getting -ve delta for sure :(

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        4 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        well cheaters ruin it for actual hard-workers and thats pretty pathetic, like when i was giving div 3's 7-8 month ago and i was rated 900, if i solved 3 i would surely get +ve, but now its sort of impossible, either you get till E to get +ve or else you get fcked

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4 months ago, # |
  Vote: I like it +7 Vote: I do not like it

bruh i asked chatgpt to solve D with dp and it actually worked

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    4 months ago, # ^ |
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    huh?? gpt might not even do it with a simple loop, how did DP work ? lmao

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      4 months ago, # ^ |
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      there is a valid dp solution

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        4 months ago, # ^ |
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        i know that brother, just stunned that gpt who cant write conditions in loop did it with DP, impressive to say the least lol

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          4 months ago, # ^ |
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          i think chatgpt is just better at dp lol

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4 months ago, # |
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thank you all for the great contest! I had so much fun solving a, b but got stuck on c :(

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4 months ago, # |
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how brute-force round ，I love it.

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4 months ago, # |
← Rev. 3   Vote: I like it +5 Vote: I do not like it

Please report this guy. He is sharing answer during the contest time. He is destroying the cp environment. here is the link : https://www.youtube.com/live/frrrP-SHXWA?si=nnX4Ghj0bqvDJEeN

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Hi, I am a newcomer here. I solved A and B in this contest but I still did not get any rating. Can anyone help me out and let me know why is that the case?

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    There is hacking phase running in this contest, you can see in the standings the time for it. You can read more about it here, once this hacking phase finishes all submissions will be run once again with new test cases that are added during the hacking phase; after it you will be able to see your rating.

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4 months ago, # |
  Vote: I like it +10 Vote: I do not like it

Fantastic round! E was an amazing problem! Thanks a lot for the round Noobish_Monk ErnKor Kmes ArSarapkin and all testers!

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4 months ago, # |
  Vote: I like it -16 Vote: I do not like it

there is a question in cf round 957 div3 just completed question no. E Novie Mistake if n==1 for a=3 and b=2 means string is empty value of n*a-b=1 but string is empty means it is zero it is not matching then why question consider string is empty as 1 and it didn't mention anywhere

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4 months ago, # |
← Rev. 4   Vote: I like it +6 Vote: I do not like it

D has a very clean DP solution.

Let $$$dp[i]$$$ denote the minimum number of swims required to reach index $$$i$$$. The transitions are -

For $$$j = i + 1 - m$$$ to $$$i$$$

  • $$$dp[i + 1] = min(dp[j], dp[i + 1])$$$, if $$$s[j] =$$$ 'L'

$$$dp[i + 1] = min(dp[i] + 1, dp[i + 1)$$$, if $$$s[i] =$$$ 'W'

Finally, the answer is YES if $$$dp[n + 1] \le k$$$.

Submission Link: https://mirror.codeforces.com/contest/1992/submission/270032188

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    4 months ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    Yeah, that's true, but I exceeded the memory limit during the contest using dynamic programming, so I used BFS instead. It is also an easy solution.

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      4 months ago, # ^ |
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      It's funny because I exceeded the memory limit using a DFS solution, and had to think of a DP solution instead.

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    4 months ago, # ^ |
    ← Rev. 3   Vote: I like it +8 Vote: I do not like it

    there is also easier greedy solution using O(1) extra memory: create a variable last_land_point and set it = i whenever you get on the land. Then if ever last_land_point become too far, that is farther than jumping distance, start incrementing a variable for how much to swim. If you hit a crocodile while swimming, not possible. If you have to swim too much, not possible.

    Code: https://mirror.codeforces.com/contest/1992/submission/269973081

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      4 months ago, # ^ |
        Vote: I like it +3 Vote: I do not like it

      That's great, bro! I liked your solution.

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Hi, I struggle with tabulation DP, can you share any resource from where I can improve ?

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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Tabulation DP is pretty much the same as recursive DP, you just have to calculate the DP values in the correct order, that is, the current state being calculated should not depend on a future state. Honestly the best way to learn tabulation DP is to solve more problems, CSES DP section the Atcoder DP contest helped me a lot.

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      4 months ago, # ^ |
      ← Rev. 3   Vote: I like it 0 Vote: I do not like it

      I would suggest Striver's DP playlist for that. If you just want to convert your recursive soln. to Tabulation then watch his videos(any of them), you will get to know that the only thing which is required for conversion is how to convert your Base cases of recursion to initialisation (base condition) of tabulation dp, rest all is just copy paste of the transition.

      And this conversion is quite easy.

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Can someone pls tell why my soln for D is giving TLE 270051817. I was trying to keep track of the indexes of logs, i tried to check if we could cover the distance with jump alone or jump+swim but i dont know why my code is giving tle. Can anyone pls help!

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it

I submitted almost correct solution for E during the contest https://mirror.codeforces.com/contest/1992/submission/270016302, the only thing I didn't figured out was to convert to long long instead of size_t in the only place to avoid overflow. So disappointed, I will NEVER use any integer types except long long in competitive programming again!

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4 months ago, # |
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Can somebody explain what's f(x) and g(x) are in '**C**', i just guessed the soln, by looking at the samples and it worked but i have no idea what is problem saying and want me to do!

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    g(i): Add up numbers in the first i positions of the list that are less than or equal to m. f(i): Add up numbers in the first i positions of the list that are greater than or equal to k.

    To maximize the value of (∑ni=1f(i)−∑ni=1g(i)), place the largest numbers (greater than or equal to k) at the beginning of the permutation to maximize f(i), and place the smallest numbers (less than or equal to m) towards the end to minimize g(i).

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4 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

// Can anybody tell me what's wrong in this implementation of D , I have chosen a variable pos to see if it is possible to pass the river and p tells if we found a "log" , c tells us if i found a "crocodile" and "w" tells us swimming through water i have as usual checked if if it is possible to get to a log then go there otherwise increase the position by m and check if there is one segment to swim to otherwise if only crocodiles were found btw i and i+m segment make pos false.





signed main() {
    ll t;
    cin >> t;
    while(t--) {
     ll n,m,k;
     cin >> n >> m >> k;
     string s;
     cin >> s;
 
    ll i = -1;
    bool pos = true;
    while(i<n){
        bool p = false;
        bool c = false;
        bool w = false;
      for(ll j = m;j>=1;j--){
          
        ll k = i+j;
        if(i+j<n){ 
            if(s[k]=='L'){
            // cout<<i+j<<endl;
            i = k;
            p=true;
            break;
         
        }
        if(s[k]=='C'){
            c = true;
        }
        if(s[k]=='W'){
            w = true;
        }
 
        }
        else{
            i = n+1;
            break;
        }
      }
 
      if(p==false and w==true){
          i+=m;
        if(i>=n){
            break;
        }
        else if(s[i]=='C'){
            pos = false;
            break;
        }
       else if(k>=1){
            // cout<<i<<"water1"<<endl;
            i+=1;
            k = k-1;
            // cout<<i<<"water"<<endl;
        }
        else{
        pos = false;
       break;
        }
      }
      else if(p==false and c==true){
        pos =false;
        break;
      }
    }
 
    if(pos){
        cout<<"YES"<<endl;
    }
    else{
        cout<<"NO"<<endl;
    }
    
      
         
 
  
}
}
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    4 months ago, # ^ |
    ← Rev. 2   Vote: I like it +5 Vote: I do not like it

    With this number of if-else-s and no proper indentation, it's hard to get the idea implemented right. The right thing to do is perhaps not to find a particular bug, but to take a step back and observe what can be changed globally. Consider developing these habits:

    • formatting the code to aid in implementation (any decent IDE has a keyboard shortcut for that)

    • thinking about solutions with as little if-else-if-if-else... forests as possible

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4 months ago, # |
← Rev. 3   Vote: I like it 0 Vote: I do not like it

Never thought i will get 4 wrong submission on Div3-A :(

However solved B,C easily but spent too much time on D and didn't got much time to solve E Seems lots of people solved D so i might get -ve delta

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I hope we get +50 delta my rating is same as yours and the same situation it was actually a speed matter because people placed at 2500 also solved 4 problems only

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I think you will reach pupil easily because you didn't participate in too much contests I participated in more contests so I would be relieved if I got those +50

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      4 months ago, # ^ |
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      nah bro just got +7, i think lot of incorrect submission for problem A cost me

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4 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Bug in https://m1.codeforces.com/enter (during contest Problem C was not able to load i shifted to main site quickly) Your text to link here...

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Can someone please help me debug this code for D ? I was able to come up with the dp idea but can't find what is wrong with the solution? https://mirror.codeforces.com/contest/1992/submission/270037120

Also, Is there some way to see the test case on which it is failing? Thank you.

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    4 months ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    Just output the testcase where it is failing.

    Spoiler
    Code
    Test Case
    Issue in the code
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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Ah Man, Thank you very much!!. Wasted 2 hours on this during the contest. But it should give some Runtime Error(Segmentation fault ) instead of Wrong Answer, Correct? Or am I missing something here?

      Anyways it was an easy problem, more than 10k submissions :O . Will get a negative delta because of this :'(

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      4 months ago, # ^ |
      ← Rev. 2   Vote: I like it 0 Vote: I do not like it

      Can you please tell the wrong test case for my code. I have been getting WA on case 559 of test 4 270114576.

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        4 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it
        Testcase
        Code

        I must say this is a bad habit is general, but this trick should be used after debugging for long time ( >= 1hr) and still the same error is not resolved.

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          4 months ago, # ^ |
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          Thanks a lot man! I have had 13 wrong submissions and never could figure out the test case where it was failing, thnx a lot. I will learn and try this trick if I struggle a lot on some task, thnx for teaching me this trick.

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4 months ago, # |
  Vote: I like it +1 Vote: I do not like it

Weird statements for D and F

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4 months ago, # |
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Does anyone know what the error-"Invalid use of deleting symbols" mean? Here's the link for reference 270073853

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    1 1 isn't valid for $$$n=1$$$, because it results in an empty string, which is exceptionally disallowed by the statement.

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4 months ago, # |
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I'm sure many people brute forced the answers of E, and just made if and else cases of it, there weren't many numbers and was fairly easy to make cases for each n.

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    4 months ago, # ^ |
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    yeah since you really only have around 50 thousand cases (optimally) and it wouldn't be very hard to just bash out all the possible cases of a and b.

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      4 months ago, # ^ |
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      No that would give TLE, I meant people brute forced the answers beforehand, and made if else cases like: if (n==2) cout << 3 << endl << x1 << x2 << endl .. ... if (n==3) ....

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        4 months ago, # ^ |
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        why would that give TLE? i did just that

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        4 months ago, # ^ |
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        Personally, I believe that it shouldn't give TLE, due to the extra second it provides for the time constraints, 3 seconds, compared to 2 in B, C, D and 1 in A. I believe it should be possible to optimize enough to fit in the time limit, as 50,000 cases really isn't all that much in computing. You do have a point though.

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          4 months ago, # ^ |
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          You are missing the factor of building the number by getting individual digits which is atleast 20 operations for each i from 1 to 10^4. So your total operations would be 10000*5*20 for each test case, so a total 10^8 operations, I personally wouldn't risk on such a solution passing the TL.

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            4 months ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            one could easily optimize this by precomputing the number of digits that u will need and build the number from there. since that number is quite small it will not TLE

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            4 months ago, # ^ |
              Vote: I like it 0 Vote: I do not like it

            I'm not sure where you're getting these numbers from. Other than the $$$n = 1$$$ case (which has $$$9999$$$ possible answers, since $$$a = b + 1$$$ always works), among all $$$2 \le n \le 100$$$ there are a total of only $$$24$$$ other $$$a, b$$$ pairs actually possible. Precomputing all these and using if else statements would definitely not TLE.

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4 months ago, # |
  Vote: I like it +12 Vote: I do not like it

really nice contest, I enjoyed the surplus of math-related questions, and it felt fitting for div 3. great job to all the organizers!

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    yeah same. as a math olympiad participant, these observation problems were easier to me, which is why i performed so well this time :D. definitely a great contest with some nice problems!

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Can someone help me why my submission give a wrong answer in problem F 270081629

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I only rated 374 and did 1 contest, this contest I did 2 problems, could I be rated in this one?

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4 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Enjoyed while solving the problems. But descriptions were just too bad. It took much time to understand each problems. Please make descriptions easy so that everybody can understand the questions easily

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4 months ago, # |
  Vote: I like it -12 Vote: I do not like it

Why did nothing happen to the cheater of the EPIC Institute of Technology Round Summer 2024 (Div. 1 + Div. 2) contest and their rating? MikeMirzayanov & Vladithur

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

Why my score can't increase after this round? In my competition record it shows unrated,why?Can anyone answer my question?My rating is still 762

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4 months ago, # |
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Congratulations to neal for taking the first place XD !!!

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4 months ago, # |
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Why my solution for F was hacked? Just because the time constant?

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4 months ago, # |
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a trend of not able to solve E and getting hacked on F continues

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4 months ago, # |
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Took me embarrassingly long and with many false attempts to solve E. Solved F and G before it.

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    4 months ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    a recent trend is to give a constructive problem as div -3,4 E making it confusing wheather to go for e or f

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4 months ago, # |
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Language of problem D was not right , you should explain all these cases properly :

1. If he is on the surface (i.e., on the bank or on a log), he can jump forward for no more than m _ (1≤m≤10_ ) meters (he can jump on the bank, on a log, or in the water). 2. If he is in the water, he can only swim to the next river segment (or to the bank if he is at the n -th meter). _3. ErnKor cannot land in a segment with a crocodile in any way.

i am able to solve it after contest by spending 2 hrs and i wasn't able to solve it during contest because langauge was not that much clear l know there are so many submissions but dont judge me by those solutions because most of them are cheaters , and its high chances that they even didnt read the problem statement

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Or you need to be not delusional and solve more dp problems.

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    4 months ago, # ^ |
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    i must agree, there were so many accepted solutions.

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4 months ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Can anyone tell me why one solution gives tle and other doesn't for F

AC : 270113651

TLE : 270113332

Not that much of a difference, in TLE one, I am erasing unique elements before I loop, Whereas in AC one I erase after I loop

UPD : I am just dumb. There was just a PEMDAS mistake XD

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    4 months ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Isn't there something similar to Set or SortedSet in c++? Why be so sophisticated.

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      4 months ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      yea there is, but this comes to my mind at the first thought during the contest as i have learned about this unique function just before the contest started. but wasn't able to implement it. Maybe just because of I was too tired after solving E

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4 months ago, # |
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I have rating less than 1600, but this contest is displaying in unrated section in my profile?

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    4 months ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    That is only because the ratings are not currently calculated

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      4 months ago, # ^ |
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      hi. i am a beginner here. just wanted to ask. now, since the hacking phase is over, by when(approximately) will the rating be shown?

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        4 months ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        After the hacking phase, all solutions will be rerun with the new test cases generated during the hacking phase. This is called System Testing. Your rating will be updated after the System Testing, but not immediately; this might take several hours even after the System Testing is complete.

        I expect the overall process to take about a day. However, note that the time required for rating updates can vary.

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4 months ago, # |
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When can the textual editorial be posted, since youtube isn't accesible in China :(

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    4 months ago, # ^ |
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    we'll try to post an editorial in the next three hours.

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4 months ago, # |
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My rating didn't change even tho I solved 2 questions?

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4 months ago, # |
  Vote: I like it 0 Vote: I do not like it

I hacked four solutions of E using the testcase where t=100 and n from 1 to 100. Now as the system tests are visible the second testcase is same as my hack test. How is this possible ?

Hack test — https://mirror.codeforces.com/contest/1992/hacks/1055241/test

Submission Link — https://mirror.codeforces.com/contest/1992/submission/270051562

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    4 months ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    It's because their solution is barely passing the time limit, so run to run variance. If it was ran again it might pass and it might not

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    4 months ago, # ^ |
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    I think test case 2 is t= 100 And n from 1 to 100

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4 months ago, # |
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I am unrated and a newbie right now but still this round is showing unrated for me..

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    4 months ago, # ^ |
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    Dude system testing hasn't started yet. The ratings dont change until its done

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      4 months ago, # ^ |
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      Thank you soo much!! Since it was my first contest I didn't know how things really work

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    4 months ago, # ^ |
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    Since you are a newcomer, this round will be rated for you. It seems you saw your rating graph with the 'All' filter. However, this does not mean that you are unrated for this round; it simply means that the ratings have not been finalized yet.

    Please wait for the system testing to be completed. After the system testing is done, you can expect your rating to be updated.

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4 months ago, # |
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Why my rating is not updated?

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4 months ago, # |
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I am a new comer and below 1600 but i didn't get any rating points and the contest was classified unrated for me, may i know why?

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4 months ago, # |
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When the results will come

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4 months ago, # |
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when is the system test?

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4 months ago, # |
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I participated in this contest and solved only 1st questions but i didnt get any ranking or even my ratings didnt get affected

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4 months ago, # |
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Why I can't be rated after this round, I just 374 rating

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    4 months ago, # ^ |
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    system test phase is happening now. Once the submissions are re judged ratings will roll out. Will take atleast few hours. You shd get the rating update by eod,

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4 months ago, # |
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I submitted A, B, and C yesterday and all three got accepted yesterday. I have no idea how B and C got rejected today ,This time i have proof also that i have done it by myself can anyone help me to reach out codeforces team Codeforces Round 957 (Div. 3)

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4 months ago, # |
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BruteForces!!!

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4 months ago, # |
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i have solved 3 problem but now only one of they remained, but why? how it is possible

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4 months ago, # |
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I had submitted 3 questions in the Codeforces Round 472 (rated, Div. 2, based on VK Cup 2018 Round 2) and it was accepted. But when I checked it just now it is not showing my submitted answers and my first question is highlighted red. Also is showing some system checking which is at 77%. Please tell me what is going on as I am new to Code Forces.

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    4 months ago, # ^ |
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    Don't worry, system testing is when all the submissions are checked again on larger amount of tests, the problems that you see "rejected" are actually just unchecked yet, you'll see the true result when system testing is finished.

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4 months ago, # |
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Is there any way I can optimise my latest solution of E problem even more ?

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4 months ago, # |
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orz

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4 months ago, # |
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Can anyone explain what is this hack doing?

https://codeforc.es/contest/1992/hacks/1054913

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4 months ago, # |
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Before the change of statement, I struggled for a long time to understand problem F as I thought about making more "good segment". It's strange, I don't know why I can't get the real meaning. Why no explanation of the problems?

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4 months ago, # |
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The jugde ends but my rating doesn't change :v

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4 months ago, # |
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May I know why this contest is not rated though am a trusted participant.

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4 months ago, # |
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how to see the test case where I Got hacked I cannot see it in hacks section, as well as there is no test case visible to me with wrong answer verdict

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    4 months ago, # ^ |
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    Just follow the instructions on the image. Also, during the hacking phase, you cannot see the hacker's input. If you can't view the tests, this might be the reason.

    Here is your test used for hack: Link

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4 months ago, # |
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Can anyone please help me correct this solution for problem D.

Got WA on 845th test of test case 2.

Code
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    4 months ago, # ^ |
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    hey buddy , the problem is the part where you define your state of dp , so what I could understand from the code was dp[i] would give me true/false stating is it possible to go to the end of the river from i.

    There are 2 things that are changing in your recursive function , that is i (index) and k (remaining swimm count) but nowhere in the dp state have you mentioned k there is just i.

    Look at this example lets say when in the first recursive call you get to an index x with remaining swim count as y and it is possible to cross the river with y amount of swimming so dp[x] will be true. but lets say in future you call the same index again with z remaining swim count (z < y) and it is not possible to swim through the river with z swim , still you will be returning "true" because dp[x] is set to true because of first function call.

    Conclusion :

    you need to change the definition of dp[i] , instead of saying dp[i] gives me true or false if it possible to reach end from index i , define dp[i] as minimum number of swimming count required to reach end of river from index i . If it is impossible to reach end from i store some large number which cant be your ans in any case. Keep overflow in mind !!

    AC soln: 270200685

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      4 months ago, # ^ |
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      Thank you, now i understand where i was going wrong.

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4 months ago, # |
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I dont see the point of adding updated testcases to a div 3. If you wish to introduce new testcases after the contest, simply make them pretests instead. It is really misleading. Yesterday my code for E got accepted with a missing condition which was compiler dependent. Due to that missing condition, the terminal would behave weirdly during runtime in the new testcases that were added. Now after the contest ended, quite obviously no one bothered to try hacking E since they had checked all values of N in the test files. It is only fair to allow the acceptance to a solution if the tests were weak and and no one hacked the solution. Had the new testcases been there since the beginning, I would have fixed the solution in the contest itself. I would request the codeforces team to look into this.

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4 months ago, # |
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Need Help ,,,, I don't understand how did I get 4 Penalty and -50 on Rating, please can anyone help me for solving this issue, I don't understand I am new here, please anyone there, hello coders

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    4 months ago, # ^ |
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    +00:04 in the ranking list does not means 4 penalty, it means you take 4 minutes to solve that question.Talking about the rating, you got negative delta bcoz you doesn't perform upto your rating level(before contest rating) in the contest.

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4 months ago, # |
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[submission:270203220]problem D

In this where i am going wrong when implementing considering state as min meters swum to reach i from 0 it is giving wron g answer i want to know about some changes

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4 months ago, # |
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how come i only get +14 rating when I solved 3 problems correctly and I am just 800 rated guy

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4 months ago, # |
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Something smells really fishy about the official winners, number 1,2 and 4..... It does not seem feasible for somebody getting a consistent rank of 5k to jump to 1, or for a 4/5 digit ranker to suddenly get a 1 digit rank in contests.....or for that matter, for a Newbie to jump straight to Specialist in one contest.....

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    4 months ago, # ^ |
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    There's almost a one-year gap between the contests where they have 5k rank and the recent ones where they have single-digit ranks so I would say it's pretty legit considering they might have been giving contests or solving problems using some alt account.

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4 months ago, # |
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hey why is my code for problem d skipped, i just prompoted gpt to make the code i give it the intuition and gpt is not banned, also this was a first time was using gpt due to less time and my many approaches having little bugs,so please look into this i am a fair user and also use sublime text which is no online ide

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4 months ago, # |
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I want to be a specialist, any tips for it,as my rating don't seem to change

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4 months ago, # |
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Anyone solved E using binary search and can you explain your thought process?

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4 months ago, # |
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4 months ago, # |
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another test

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4 months ago, # |
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Where's my rating?

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4 months ago, # |
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I hope this message finds you well. I recently received a notification stating that my solution (ID: 269986506) for problem 1992D significantly coincides with the solutions submitted by users abhishekgoyal11aug2003/269986506 and TLE_lord/270033963.

Here is the code I submitted:

Copy code

include <bits/stdc++.h>

typedef long long int ll; typedef unsigned long long int ull; typedef long double ld;

define mod 1000000007

using namespace std;

int main() { ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL); ll t; cin >> t; while(t--) { ll n, k, m; cin >> n >> k >> m; string S; cin >> S; string s = 'L' + S + 'L'; ll swim = 0; bool canSwim = true; ll len = n + 2;

for (int i = 0; i < len; i++) {
        if (s[i] == 'C') {
            canSwim = false;
            break;
        } else if (s[i] == 'L') {
            ll l = -1;
            ll w = -1;
            for (int j = i + 1; j <= min(len-1, i + k); j++) {
                if (s[j] == 'L') {
                    l = j;
                } else if (s[j] == 'W') {
                    w = j;
                }
            }
            if (l != -1) {
                i = l - 1;
            } else if (w != -1) {
                i = w - 1;
            }
        } else {
            swim++;
        }
    }

    if (swim > m || !canSwim) {
        cout << "NO" << endl;
    } else {
        cout << "YES" << endl;
    }
}

} I implemented a standard greedy approach in this solution. Upon reviewing the user TLE_lord’s submissions, I found that there were nearly 8 submissions, with significant time differences between them. I submitted my solution almost an hour ago, and it seems likely to be a coincidence rather than plagiarism.

I kindly request you to review this matter and rectify any discrepancies at the earliest convenience.

Thank you for your attention and assistance.

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