diskoteka's blog

Codeforces Round #894 (Div.3)
By diskoteka, 3 years ago, translation, In English

☀ ☀ ☀ Hey Codeforces ☀ ☀ ☀

Summer is ending, but my team and I are happy to invite you to participate in Codeforces Round 894 (Div. 3). The round will take place on Aug/24/2023 17:35 (Moscow time). You will be offered 7 problems with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have a rating of 1600 or higher, can register for the round unofficially.

The round will be hosted by rules of educational rounds (extended ICPC). Thus, solutions will be judged on preliminary tests during the round, and after the round, it will be a 12-hour phase of open hacks.

You will be given 7 problems and 2 hours and 15 minutes to solve them.

Note that the penalty for the wrong submission in this round is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory 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, then the round will be rated for you.

Problems have been created and written by: diskoteka, pavlekn, playerr17. Ivang helped us with the idea of one of the tasks.

We would like to thank:

  1. Vladosiya for coordinating the round

  2. MikeMirzayanov for great Polygon and Codeforces platforms

  3. feeder1, Xellos for red testing

  4. michao, induk_v_tsiane, Phantom_Performer, vladmart, dmkozyrev, LordVoIdebug, vrintle for yellow testing

  5. kzyKT, cel.in for purple testing

  6. Zixel, Egorsa, MADE_IN_HEAVEN, Thost, FiniteMoves, mewnya, natalina, Insightful for blue testing

  7. artew, tnaito, akwa_blue, Zeyad_Hekal, NgJaBach for cyan testing

  8. mkshh, myav for green testing

We wish you all good luck and a high rating!

UPD: Editorial published

  • Vote: I like it
  • +370
  • Vote: I do not like it

Comments Comments (187)
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3 years ago, hide # |
 
Vote: I like it -7 Vote: I do not like it

As a tester, I really liked the problems. I wish you good luck and have a nice day!

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3 years ago, hide # |
 
Vote: I like it +6 Vote: I do not like it

As a participant I will participate and be pupil again: : : : :

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3 years ago, hide # |
 
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do not have a point of 1900 or higher in the rating.

Should it be 1600?

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    3 years ago, hide # ^ |
     
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    idk why, but it was always 1900

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    3 years ago, hide # ^ |
     
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    no

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    3 years ago, hide # ^ |
     
    Vote: I like it +16 Vote: I do not like it

    I guess you don't understand what that part means. First, you should probably read the last paragraph of this blog.

    From the blog:

    -- We will exclude from the official standings of Div. 3 rounds and put in a separate rooms all those who can’t be reliably called to be a real participant. Accounts that materially participated in less than 2 rating rounds (materially means solved at least one problem there) before the start of the Div. 3 rounds, and those who have ever gained 1900 or more rating units will not get into the official standings and will be assigned to separate rooms. However, this does not mean that there is no rating recalculation for them. Thus, the rating will be updated for all users whose rating is strictly less than 1600 at the time of the start of a round.--

    In short, everyone who has $$$ \lt 1600$$$ rating will receive rating change after the round, but only some of these participants will be shown in the "official standings". Namely,

    • People who have participated in less than $$$5$$$ rated contests (originally $$$2$$$ as you can see) are excluded from the official standings.
    • People who have reached $$$\ge 1900$$$ rating, and later dropped back to $$$ \lt 1600$$$ rating are excluded from the official standings.

    These rules are in place to try to make sure that everyone in the "official standings" is actually at Div.3 skill level and not someone much better just having low rating. I think it is more fair to make this rating upper bound at $$$1900$$$ than $$$1600$$$, since reaching $$$\ge 1600$$$ rating doesn't mean you are actually that good (you could've gotten lucky).

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3 years ago, hide # |
← Rev. 3  
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As a tester, I demand my name in the blue testing list and can assure that problemset is very well balanced and no problem is a cakewalk and wish everyone good luck and recommend to read all the problems

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3 years ago, hide # |
 
Vote: I like it +15 Vote: I do not like it

What about Vika from your last contest

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3 years ago, hide # |
 
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I have the impression that the contest would take place on 8/26 (Sat.) Has it really been moved up, or did I get the date wrong?

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3 years ago, hide # |
 
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I hope I can reach Master AFTER this round :v

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3 years ago, hide # |
 
Vote: I like it -8 Vote: I do not like it

As a participant I will participate hoping to be an expert XD

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3 years ago, hide # |
 
Vote: I like it +3 Vote: I do not like it

Sauce for artwork

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3 years ago, hide # |
 
Vote: I like it +5 Vote: I do not like it

Wish to be blue !!!

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    3 years ago, hide # ^ |
     
    Vote: I like it 0 Vote: I do not like it

    I'm really grateful for the opportunity this contest gave me to reach the blue！

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3 years ago, hide # |
 
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I hope to be blue after this contest.

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3 years ago, hide # |
 
Vote: I like it +6 Vote: I do not like it

Fingers crossed that this time around, the problems are easy to understand unlike your previous round.

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3 years ago, hide # |
 
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Is that the girl in the blog Vika, which you mentioned in your last contest? Whoes stories were hard to decode >_<

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3 years ago, hide # |
 
Vote: I like it 0 Vote: I do not like it

How is it going with Vika

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3 years ago, hide # |
 
Vote: I like it +21 Vote: I do not like it

No offense, but pls no Vika in this round

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3 years ago, hide # |
 
Vote: I like it +3 Vote: I do not like it

I am crying because of interactive problem will be in contest.

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3 years ago, hide # |
 
Vote: I like it +13 Vote: I do not like it
Heyy you
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3 years ago, hide # |
 
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hope I'll regain blue soon...

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3 years ago, hide # |
 
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that's not yellow it's orange sir.

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3 years ago, hide # |
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.

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3 years ago, hide # |
 
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What does red/green/etc. testing mean? I thought, tests are the same for everybody

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3 years ago, hide # |
 
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Hoping for great div3 contest__

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3 years ago, hide # |
 
Vote: I like it 0 Vote: I do not like it

Yall will like the problems.

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3 years ago, hide # |
← Rev. 8  
Vote: I like it -27 Vote: I do not like it

.

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    3 years ago, hide # ^ |
     
    Vote: I like it 0 Vote: I do not like it

    Did you even read the announcement?

    If your rating is less than 1600, then the round will be rated for you.

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3 years ago, hide # |
 
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Hope my color will change after this round . Lets go !

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    3 years ago, hide # ^ |
     
    Vote: I like it +1 Vote: I do not like it

    It will...... .......................................... .......................................... ............................ ............................ ............................ ...................... be green hh

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3 years ago, hide # |
 
Vote: I like it 0 Vote: I do not like it

Really hope to reach pupil this time!

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3 years ago, hide # |
 
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good luck for everyone :)

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3 years ago, hide # |
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Vote: I like it -33 Vote: I do not like it

:(

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3 years ago, hide # |
 
Vote: I like it +3 Vote: I do not like it

I can't wait, I'm so excited about Div 3

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3 years ago, hide # |
 
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So many Yellow and Blue testers for a cyan, green & grey round

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3 years ago, hide # |
 
Vote: I like it +2 Vote: I do not like it
Hope to Solve 3 Problems
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3 years ago, hide # |
 
Vote: I like it +11 Vote: I do not like it

diskoteka how is your vika now ? after reading last contest's problem statements

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3 years ago, hide # |
 
Vote: I like it +2 Vote: I do not like it

Expert round?

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3 years ago, hide # |
 
Vote: I like it +1 Vote: I do not like it

35k participants

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3 years ago, hide # |
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why 'custom invocation' not working ? (: diskoteka

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    3 years ago, hide # ^ |
     
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    he can't control that , even mike can't , it's not working as it is contest time and too much pressure on the judge

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3 years ago, hide # |
 
Vote: I like it +12 Vote: I do not like it

Good contest, enjoyed it !!

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3 years ago, hide # |
 
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when it's bout div.3, diskoteka never disappoint us.

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3 years ago, hide # |
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gg, this was my first contest and i didnt know what to expect from it. The questions were great, but my solutions aren’t(had to scour over StackOverflow for a few places...). but congratz!

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3 years ago, hide # |
 
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For some reason my solution to A got all different outputs than what I got when I ran it on my PC, lost 2 hours because of that, is that normal during contests or was that some kind of bug? I switched to C#, wrote the exact same code and it got accepted...

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    3 years ago, hide # ^ |
     
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    Your uzeli array is uninitialised so has random undefined contents on each run.

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      3 years ago, hide # ^ |
       
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      then why did it always work on my PC? So basically I had to set all values of that array to false?

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        3 years ago, hide # ^ |
         
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        You got lucky I guess (different compiler / optimisation settings may produce different results). It isn't worth trying to work out why it works sometimes when you have undefined behaviour, just fix the undefined part and it'll work everywhere.

        if you declare it as bool uzeli[m]{}; instead of bool uzeli[m]; it will be default initialized (which for bools is all false).

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3 years ago, hide # |
 
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Having spent lots of time for E because of the third test case, that's really confuse because of the note. I got AC because of predicting the actual problem LOL.

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3 years ago, hide # |
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what is error in this code For Problem F https://mirror.codeforces.com/contest/1862/submission/220289269

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3 years ago, hide # |
 
Vote: I like it +6 Vote: I do not like it

mid=(lo+hi)/2 forces

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    3 years ago, hide # ^ |
     
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    Yes solved the problem D with quadratic and got WA, and I know it time to use bi-search

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      3 years ago, hide # ^ |
       
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      D can be solved without binary search.

      Here's my submission — 220221455

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        3 years ago, hide # ^ |
         
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        can you please explain you solution. thanks in advance :)

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          3 years ago, hide # ^ |
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          If you observe carefully there is a pattern in the solution — 2,3,3,4,5,4,5,6,7,5... So it is like -

          2,3

          3,4,5

          4,5,6,7

          5,6,7,8,9

          and so on

          If you solve the equation nC2=r, where r=required number of different ice creams, you will get n = (1+sqrt(1+8r))/2, so just calculate this value if it is an integer it will be your answer, and if not, then it will be the first number of the row in which your answer is present so just add the diff between r and the value for which you got the answer which you can find by simply putting it back in the formula nC2 and that will be your answer.

          Sorry if I was not clear, but the main thing is finding the pattern and the quadratic then you can solve this question.

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            3 years ago, hide # ^ |
             
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            bro but in sample test case 4 for 179 why do we need 27 , by nc2 it can be wo also right because 20 c2 is 190 more than 179

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              3 years ago, hide # ^ |
               
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              In question D. it is given that exactly n type of ice-cream has to be made, not greater than that not less than that. for test case 4 answer is 27 because you need 19 unique ice-cream flavor and 8 repeating flavor. e.g. 1,1,2,2,3,3,..

              19c2 = 171.

              since you have repeated 8 flavor you can make 8 unique combinations of (1,1),(2,2) ....

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              3 years ago, hide # ^ |
               
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              That’s what I thought too. I spent like half an hour tryna understand until the general announcement then got AC right away

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    3 years ago, hide # ^ |
     
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    Extremely short solution without binary search: https://mirror.codeforces.com/contest/1862/submission/220208197

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3 years ago, hide # |
 
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I cannot get the test details in E, can you help me what wrong with my code ? This is my code

:(( I know I cannot get more rating after this contest :(

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3 years ago, hide # |
 
Vote: I like it +13 Vote: I do not like it

Thank you Geothermal for this trick while using bitset.

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3 years ago, hide # |
 
Vote: I like it +2 Vote: I do not like it

I don't know if problem C was actually confusing or maybe I'm dumb enough not to understand it.

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3 years ago, hide # |
 
Vote: I like it +1 Vote: I do not like it

Can someone hack this F solution for the case when water and fire > 1e5? https://mirror.codeforces.com/contest/1862/submission/220284156

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3 years ago, hide # |
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for D i noticed it's . find the nearest lowest X for X choose 2 then add n-(X choose 2) . but since it failed on the last test since it's a very big number so is my logic correct and bad implementation? Edit: ok nvm i was an idiot and made r in the binary search n not 1e9 :).

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Good problems!Thank you! And I want to know is there any brute force ways for F?I got wa using my brute force way :(

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3 years ago, hide # |
 
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Problem D:

I used binary search on k, trying k choose 2. If I found kc2 == n, answer is k.

If such a k does not exist (for example, n = 7), I found the greatest k such that kc2 < n, and then added duplicate balls. So for 7, 4 choose 2 is 6, add 1 duplicate ball (e.g. {1, 1, 2, 3, 4}) to get 5 as the answer.

This strategy worked for test 1 but not test 2. During the contest I believed it was a binary search bug but now I wonder if this strategy is valid.

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3 years ago, hide # |
 
Vote: I like it +1 Vote: I do not like it

Just got F a moment after the contest ...

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3 years ago, hide # |
 
Vote: I like it +1 Vote: I do not like it

Here is my Live Screencast of solving problems [A -> F] (with commentary in HINDI).

PS: Don't judge me by my current rating :(

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3 years ago, hide # |
 
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Nice problemset. Thanks for listening to the feedback on last round and made your contests better . Loved D

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3 years ago, hide # |
 
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Can someone hack my F. 220282299

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3 years ago, hide # |
 
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How could problem E be approached? What I observed is that we optimally must select the shortest continuous subarray starting at 0 because we will always remove -d * len(a), where a is that subarray. Then we can pick maximum $$$m$$$ positive values, but how to determine the length of that subarray? Would a priority queue work and at each index check the maximum m values + -d * i?

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    3 years ago, hide # ^ |
     
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    You don't need the shortest continuous subarray. The answer is maximum of: for each position sum of the greatest m positive elements to the left minus the position multiplied by d. If you get more than m positive elements to the left, you just erase the minimum of the set or the priority queue and update the sum.

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3 years ago, hide # |
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F is an interesting dp problem. Unfortunately, I was not able to solve it for the limited time.

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My F solution was Iterative dp with memory optimisation

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3 years ago, hide # |
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lognforces

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220296987

What is making my solution slow for G? (time limit exceeded) I would want to hear some advices on how to use sets and what to avoid to not get tle in the future.

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3 years ago, hide # |
 
Vote: I like it +9 Vote: I do not like it

Nice problemset.

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Most confusing problem "D" :) && Besides lots of announcement increases the confusion level further except clearing.. ConfusingForces

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    3 years ago, hide # ^ |
     
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    3 years ago, hide # ^ |
     
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    Hey Cheater, why have you inserted unnecessarily string/char etc. in your code of D.

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      3 years ago, hide # ^ |
       
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      Could you point out the unnecessary string/chars you saw in his code? If you are referring to blank lines, they don't matter anyway. Could you clarify what part of the code made you believe the person you accused a cheater is actually a cheater?

      Code for reference
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        3 years ago, hide # ^ |
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        Nvm. Very my fault for commenting without checking the problem of the solution. Thought it was this Contest's D.

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          3 years ago, hide # ^ |
           
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          I am not sure if you and the accuser are the same person. But accusing someone a cheater is not something we should take lightly. Your reasoning of accusing someone cheater because you thought the code was for this contest is not only unacceptable but also unforgivable. At least check the problem ID before calling out someone a cheater. Anyway, I hope you have learnt from your mistake and would take this seriously in future.

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            3 years ago, hide # ^ |
             
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            I didn't accuse him. I literally specified it in the previous version that I neither accuse nor defend.

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              3 years ago, hide # ^ |
               
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              If "Hey Cheater" is not accusing someone a cheater then I don't know what does accusing means. Anyway, I am not quite sure why are you replying? Are you and ccpp same person or distant cousins?

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                3 years ago, hide # ^ |
                 
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                I have no idea who he is, but I didn't say "hey cheater" in any of my comments, in the CF history of this account.

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                  3 years ago, hide # ^ |
                   
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                  My bad then, I think you should have replied in the separate thread. Cause, my question was specifically to ccpp. And I did not get any answer. Your comment confused me.

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                  3 years ago, hide # ^ |
                   
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                  Sorry, Actually I went to saw Sol of D in his profile (submissions) then I saw this soln. I did not realize that the soln was od another contest and presumed that he had done it to pass plagrasims. I am extremely sorry to have created this confusion.

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      i knew that grays dont have brains. i was unaware that they cant even see properly

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    Extremely sorry bro, I mistakenly thought you cheated. Sorry please forgive me. Sorry Again.

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      It's Ok bro.. Understanding the coding aspect is enough.

      That's actually not unnecessary strings/char. That's called template. We mainly use it for writing code faster in contest time, Cause you already know your coding speed will impact your contest performance

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My solution for F is slight different from others, I binary searched on time, and got amount of fire and water. Now if any of these is greater than sum, this time works. Else, I try to fill knapsack of fire in most optimal way, (by optimal I mean, filling it in such a way that it has least remaining space possible) and check if remaining sum can be filled in water knapsack. Similarly, tried to fill water knapsack in most optimal way and check if remaining can be filled in fire knapsack.

Here is link to submission

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    if((sum-dp[F]<=W)||(sum-dp[W]<=F))

    You only need to check one of these. Suppose sum-dp[F] <= W is true:

    dp[F] is the sum of a subset of monsters sum - dp[F] is the sum of the remaining set of monsters

    Since sum - dp[F] <= W, we can kill the remaining set of monsters with W. In other words, sum - dp[F] <= dp[W].

    Therefore, sum - dp[W] <= dp[F] <= F and the second statement is true as well.

    The same argument applies if you want to prove the second statement implies the first.

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    i did the same

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Hello, can someone provide a hint for problem E

Thanks :pray:.

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    Lol i thought it was question F nvm mb

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    hint 1
    hint 2

    Have a good day!

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Video editorial for problems A&B&C&D: https://youtu.be/aEB8NXX-lxA

Thought would be useful

IN ENGLISH

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3 years ago, hide # |
 
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Great problems, D and F were really interesting, and E was also nice. Hope to get to expert after the rating changes :)

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3 years ago, hide # |
 
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In problem F, I've seen many solutions that use this bitset idea, but I'm just lost at what it does.

Can someone explain what it's supposed to be happening here?

bitset<1000001> dp;
dp.set(0);
for (int j = 0; j < n; j++){
    dp |= dp << s[j];
}
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    3 years ago, hide # ^ |
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    this is the equivalent of

    for i from 1 -> n:

    for j from sum -> a[i]:

    dp[j] |= dp[j — a[i]]

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    3 years ago, hide # ^ |
     
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    generates all subset sum which are possible.

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    3 years ago, hide # ^ |
     
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    OK, I think I got it!

    For the interested:

    dp[amount] should be $$$1$$$ when we can form amount by summing some elements of s together, and $$$0$$$ when we cannot.

    Therefore dp should have enough size to store all possible different amounts. In this case, from $$$0$$$ to $$$10^6$$$.

    Initially dp will have all values set to $$$0$$$ (we don't know yet which values we can form), except for the $$$0$$$-th bit (we always can sum up to $$$0$$$, just choose no elements from the set).

    The line that sets the $$$0$$$-th bit to 1 is: dp.set(0).

    Then we iterate over all elements of our set. Say that our set s is {3, 2}.

    In the first iteration of the for, we know that we can form s[0] — that is, when we choose only the first element to form our sum — so we should set the s[0]-th bit of dp to $$$1$$$.

    After the first iteration, the $$$3$$$-rd (and $$$0$$$-th) bit of the bitset will be $$$1$$$:

    dp equals 00000 ... 0001001.

    In the second iteration, when considering s[1], we know that for each value we already know we can form, we can also form that value + s[1]. Since each bit already set in dp represents a value we know we can form, by left shifting each bit $$$1$$$ of dp by s[1], we get all new values we can form.

    In the example, since s[1] is $$$2$$$, by shifting all bits of dp by $$$2$$$, we have:

    dp << 2 equals 00000 ... 0100100 (now we know we can form 2 and 5).

    By using | between dp and dp << 2, we join all the values we already knew we could form and these new values we just learned we can form: dp |= dp << s[1].

    dp |= dp << 2 equals 00000 ... 0101101 (we can form 0, 2, 3 and 5).

    The same idea for s[1] is valid for any s[i] that comes after, and after the for, we know all values from $$$0$$$ to $$$10^6$$$ that can be formed, since they're set to $$$1$$$ in the bitset.

    There are probably better and shorter explanations elsewhere, but since this is a Div3, I think it means no harm putting mine here.

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3 years ago, hide # |
 
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It's very unlikely that 1000 people will solve F during contest, I think heavy cheating is going on maybe those Unofficial Experts , CM, M.. provide these fellow cheaters the solution :(

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    3 years ago, hide # ^ |
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    F wasn't that hard to be honest. I needed around 30 minutes to solve it(spent 10/15 minutes on E after solving C). I immediately had the binary search plus knapsack idea. But wasn't really sure if it would pass(I was thinking about normal knapsack not the bitset one). Then I was able to reduce it to only knapsack without the binary search.

    I participated virtually though!

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      3 years ago, hide # ^ |
       
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      What's the t.c. of bitset idea? Is it O(sum + n)

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        3 years ago, hide # ^ |
         
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        O((n * 1e6) / 32 + n)

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          3 years ago, hide # ^ |
           
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          can you explain it please.

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            3 years ago, hide # ^ |
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            performing an operation bitset has a time complexity of n / 32 or n / 64 depending on your computer, but in codeforces it seems like it is 32.

            we iterate through every number, then we take the whole bitset and we perform OR operation with (bs << val), this takes O(ceil(SIZE / 32)), SIZE in this case being sum which is 1e6.

            we do this n times, which gives us the total time complexity of O(n * ceil(SIZE / 32)) or O(n * 1e6 / 32)

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    3 years ago, hide # ^ |
     
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    I was also surprised

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Python users, for Problem G anyone knows the source of this SortedList template 220199018? (huangxw, yuki_keshiki)

I was getting TLE with the PyRival implementation.

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    I copy the SortedList template from someone's submission in Atcoder contest，it is crazy fast，prefect to solve this problem.

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White spaces and endl seem no difference to checker. 220288316

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Can problem E be somehow solved using binarysearch?

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Hey everyone, I'm a newbie. I want to ask why this round is not rated for me. Thanks.

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the observation, maximum difference between adjacent elements of the array for problem G is damn cool !!

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Why are the rating changes not out yet, hacking is over right

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nice contest i like it , F problem is so goood

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where is the systerm test?

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Can E be done using sliding window?

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Hope to reach pupil after this contest. Solved 4 problems

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Why don't start system test immediately ?

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Why don't start system test immediately!!!!!

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Why don't start system test immediately ?

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I am a python user but I dont know what is the alternative of multiset of C++ in python?

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Good round in total, but problem A is very boring, problem D and F are a little boring, the others are interesting!

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Now i have become expert the destroyer of division 3.

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commented on a wrong round sorry

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Lmao I binary searched on F and created a dp table every time not realizing I could've just created a big one to start. Oops.

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Attention!

Your solution 220250441 for the problem 1862E significantly coincides with solutions wont_give_srsly/220246281, manvendrasinghshekhawat/220250441, bdyby11/220257824, pmqwerty/220262992, Swarupa/220264041, pranavkumarreddy567/220268055, DarkDreamin/220269225, nitiksharma/220269800, jvamshi36/220271912, jolly_79/220273460, Tintintani/220275635, absolute_07/220277807, jkoushik_iiitn/220278018, abhay5a/220278945, anjan_reddy/220279911, kunaltelangi/220280520, Mobbbbb/220281738, i_decided/220284890, Adityaraj834/220285184. Such a coincidence is a clear rules violation. Note that unintentional leakage is also a violation. For example, do not use ideone.com with the default settings (public access to your code).

I recently received this mail stating that I was accused of plagiarism for a problem 1862E. I didn’t know any of these people nor did I share my code with anyone or use any online ide. I am 100% sure that this violation has not happened from my end.

I have personally looked at all the other submissions and none of them match exactly with mine other than using ‘multiset’ and the problem logic. I request you to please check again MikeMirzayanov.

My Submission: 220250441

Other Submissions:

220246281 220257824 220262992 220264041 220268055 220269225 220269800 220271912 220273460 220275635 220277807 220278018 220278945 220279911 220280520 220280520 220281738 220284890 220285184

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3 years ago, hide # |
 
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please update the problem ratings, it has been more than 20 days since the contest is finished and yet problem ratings are not updated.

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