flamestorm's blog

Codeforces Round 937 (Div. 4)
By flamestorm, 8 months ago, In English

Hej, Codeforces!

mesanu, SlavicG and I are very excited to invite you to Codeforces Round 937 (Div. 4)! It starts on Mar/28/2024 17:45 (Moscow time).

The format of the event will be identical to Div. 3 rounds:

  • 5-8 tasks;
  • ICPC rules with a penalty of 10 minutes for an incorrect submission;
  • 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
  • by default, only "trusted" participants are shown in the results table (but the rating will be recalculated for all with initial ratings less than 1400 or you are an unrated participant/newcomer).

We urge participants whose rating is 1400+ not to register new accounts for the purpose of narcissism but to take part unofficially. Please do not spoil the contest for the official participants.

Only trusted participants of the fourth 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 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 1400 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 1400 (or you are a newcomer/unrated), then the round will be rated for you.

Thanks a lot to the testers: erekle, vladmart, nskybytskyi, Vladosiya, KrowSavcik, Dominater069, LucaLucaM, MADE_IN_HEAVEN, tvladm, nor.

We suggest reading all of the problems and hope you will find them interesting. Good luck!

UPD: The round is delayed by 10 minutes: https://mirror.codeforces.com/blog/entry/127616?#comment-1133556.

UPD: The editorial is posted!

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8 months ago, # |
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i wish i could solve all problems glhf

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8 months ago, # |
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As a participant,I would like to thank for new Div 4 contest

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8 months ago, # |
  Vote: I like it -9 Vote: I do not like it

Dominater069 orz

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8 months ago, # |
  Vote: I like it -7 Vote: I do not like it

hope to reach CM

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    7 months ago, # ^ |
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    Are you stupid or something, how are you planning to reach cm.

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8 months ago, # |
  Vote: I like it +25 Vote: I do not like it

Why no green/cyan testers in Div4 round?

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

Hope everything goes OK!

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8 months ago, # |
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Ramadan Kareem

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8 months ago, # |
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Hope this will be the best Div.4 ever

Good Luck for every parcipicant!

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

Div 4 : The format of the event will be identical to Div. 3 rounds Div 3 : The format of the event will be identical to Edu rounds Edu rounds: The format of the event will be identical to Icpc rules

Is that a recursion?

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8 months ago, # |
  Vote: I like it +13 Vote: I do not like it

Hope it isn't a div2.5 as last time

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8 months ago, # |
← Rev. 2   Vote: I like it +15 Vote: I do not like it

Ladies and gentlemen, finally I can say what I've always wanted to say at Div. 4 rounds announcements: My first unrated contest :")

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    8 months ago, # ^ |
    ← Rev. 6   Vote: I like it +12 Vote: I do not like it 
    "Ladies and gentlemen, finally I can say what I've always wanted to say at Div. 4 rounds announcements: My first unrated contest :)"
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8 months ago, # |
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My second unrated div 4 contest :-)

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8 months ago, # |
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Eagerly waiting for some "lovely" div4 questions , Cyan is on!

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

please be a div 4 contest for div 4 people

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8 months ago, # |
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Hopefully, My last rated div4

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8 months ago, # |
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Hope efforts pay and this will be my last rated div 4

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8 months ago, # |
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8 months ago, # |
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The first unrated div.4.

GL&&HF

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8 months ago, # |
  Vote: I like it -6 Vote: I do not like it

my name is nate higgers

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8 months ago, # |
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why do codeforces change time? i got email that contest will start at 14:35. Now it will be 20:35

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    8 months ago, # ^ |
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    No,it starts in the usual time

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      8 months ago, # ^ |
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      Thursday, March, 28, 2024 14:35 (UTC) they send it like that

      For ex: My country's in (UTC+4) that's why it starts at 18:35

      In your country, it will start (UTC+6)- it starts at 20:35

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

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8 months ago, # |
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Hope to become Pupil with this contest :D

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    8 months ago, # ^ |
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    Solve more problems with less penalty

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      8 months ago, # ^ |
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      Managed to solve 4 without penalty today

      and -6 on Q-E :)

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        8 months ago, # ^ |
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        Nice, keep practicing. I got 5 wrong submission, and all are on test 1. -.-

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8 months ago, # |
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Why there isn't any gray,green or cyan tester?

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8 months ago, # |
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I hope I can go green after this game.

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

where are "one refresh costed me 10 minutes" comments?

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8 months ago, # |
  Vote: I like it +142 Vote: I do not like it

Sorry, but I'm moving the round 10 minutes forward. The training session was very cool and long. I'm running to you from the gym!

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8 months ago, # |
  Vote: I like it -21 Vote: I do not like it

delayforces

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8 months ago, # |
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The contest is delayed?

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8 months ago, # |
← Rev. 2   Vote: I like it -19 Vote: I do not like it

.

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8 months ago, # |
← Rev. 2   Vote: I like it -27 Vote: I do not like it

.

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8 months ago, # |
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Hey guys, anyone have the problems for this div 4 so I can practice beforehand?

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8 months ago, # |
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and i was thinking why i am not able to see problems

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8 months ago, # |
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why it was delayed? :O

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8 months ago, # |
  Vote: I like it -13 Vote: I do not like it

Aiming for puple, wish me luck guys!!

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8 months ago, # |
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The first contest after registering my account many years ago.

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

queueforces ?

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    8 months ago, # ^ |
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    queueforces this happens every first 10 minutes of a div4/div3 man

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8 months ago, # |
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Seems like div5

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8 months ago, # |
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today I broke my record. solved 3 problems :)

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8 months ago, # |
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A,B,C,D,E Screencast: https://youtube.com/live/n4f4htV009w

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

nice problem set , thanks to the authors

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8 months ago, # |
  Vote: I like it +20 Vote: I do not like it

It's fun that due to the long queueforces (and my own laziness to open a proper local IDE) I would actually submit all problems in blind and fix them accordingly after every CE/WA/RTE/TLE/MLE...

What was I doing with my life...
Thoughts on problems
Hints on problems
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8 months ago, # |
← Rev. 2   Vote: I like it -7 Vote: I do not like it

Wow only me ig, average python user

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8 months ago, # |
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What was the idea behind F?

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

How to do F in one line

Spoiler

Looks pretty weird but got AC by the way =))

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    8 months ago, # ^ |
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    can you explain furtherly the thought process behind that?

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      8 months ago, # ^ |
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      Basically the construction idea is identical to the editorial, so you should read it first. I will summarize a little bit.

      If a does not equal to c — 1, then there is no answer. Otherwise, we will build a full binary tree from top to bottom, using the nodes 2 and nodes 1. The height of this tree is p = ceil(log2(c)). After that we will calculate the number of "missing" nodes in the last level of the tree. It will be $$$2^p - c$$$. Finally, we will add the nodes 1 into the last level of the tree, level by level, as it will minimize the height of the tree.

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8 months ago, # |
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Fast tutorial

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8 months ago, # |
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do I have to participate in 5 contests to receive a rating? do all 5 contests have to be div.4??

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    8 months ago, # ^ |
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    Just 5 contests, the division does not matter

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      8 months ago, # ^ |
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      if I do 5 div4/div3 is my rating going to be more accurate/ higher? What do you recommend?

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8 months ago, # |
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In the time converter problem, latest input data can't be a coincidence. I wish I could know what was the inspiration behind this particular input data. 4 problems solved, but stared late today, I hope for a green field.

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8 months ago, # |
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A -> just do what is asked ( i had 1 wrong submission in it, forgot take input a , b , c ;( )

B -> just do what is asked pattern printing

C -> standard question leetcode style

D -> here i generated all the possible product possible, then counted whether they exist in my set or not

E -> string hashing to check wether string are equal or not ( wasted lot of time in implementing string hasing i am stupid )

F -> ran out of time

G -> ran out of time

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    8 months ago, # ^ |
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    I don't think hashing is necessary to solve E

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    8 months ago, # ^ |
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    you can solve E without hashing first the upper bound for number of divisors will be (n ^ 1/3) then all u need is to try all divisors of n for each divisor you can check if k is valid in o(n) by grouping indeces(i, i + k, i + 2k , ...) for each i <= k then you its easy to check wither its valid or not thats overall complexity o(n * n^ (1/3))

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    8 months ago, # ^ |
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    You don't need string hashing in problem E.

    The idea is

    • You just need to check lengths that are divisors of n (This will be a O(sqrt(n)) loop).

    • Now for each of the length, it will be a linear O(n) check to see if it's valid (I will leave the logic for you to think about...)

    • The total time complexity is O(n * sqrt(n))

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    8 months ago, # ^ |
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    somehow i thought that listing all the possible binaries in D and then using dfs was a good idea i am stupid

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    8 months ago, # ^ |
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    Bro thinks he's a red coder

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      8 months ago, # ^ |
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      does it matter, if i am red coder or not, if I share my thought process in comments

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8 months ago, # |
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Why does these 253813411 gets tle even though it looks to me as O(n*2^n)

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    8 months ago, # ^ |
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    It seems like you are trying all possible routes, which I'm afraid would be $$$\mathcal{O}(n!)$$$.

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      8 months ago, # ^ |
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      Thanks for your reply,but why does the initution fail that at every vertex we have two possibilities either to take it in our path or avoid it hence total 2^n operations in recursion

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        8 months ago, # ^ |
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        Not quite. Assuming a complete graph of $$$n$$$ vertices, your DFS would function something like this:

        dfs(1,0) -> dfs(2,1) -> ... -> dfs(n-2,n-3) -> dfs(n-1,n-2) -> dfs(n,n-1) -> dfs(n-1,n-2) -> dfs(n-2,n-3) -> dfs(n, n-2) -> ....

        You could guess it. It goes back to n again, but from a different parent, to seek a different path.

        Your intuition neglected a fact that ordering also matters here, and assuming a full path is required always, each path will be a permutation of those $$$n$$$ vertices, implying the actual order in the path.

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          8 months ago, # ^ |
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          Thank you very much!

          Can you give a hack idea please 253831864

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

            Hacked.

            Input:
            Idea
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              8 months ago, # ^ |
                Vote: I like it +31 Vote: I do not like it

              Cam ơn rat nhieu :)

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                8 months ago, # ^ |
                  Vote: I like it +31 Vote: I do not like it

                I'm surprised you actually showed your appreciation in my native language.

                आपका स्वागत है।

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8 months ago, # |
  Vote: I like it +13 Vote: I do not like it

Good competition! Thanks flamestorm,mesanu, SlavicG, and the testers :)

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8 months ago, # |
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Hey guys, I solved 4 problems, A, B, C, E. I see that each of my solutions got accepted bu only with one test. So, how can this be? How can a solution be right with only one test. Will it be counted as wrong later?

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    8 months ago, # ^ |
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    of course, if after contest you WA in another test, you will get red -1 and drop rating

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8 months ago, # |
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interesting round and one of the best div 4's,keep it up.

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8 months ago, # |
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Who approved g? Why?

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    8 months ago, # ^ |
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    A NP-hard problem. LOL

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      8 months ago, # ^ |
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      I submitted a n^2 2^n solution which later got hacked due to TLE. Any idea what was the intended time complexity/approach?

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        8 months ago, # ^ |
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        It was the intended complexity. I assume that you were hacked because of unordered_map instead — this has something to do with hash tables' collisions.

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          8 months ago, # ^ |
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          Oh my previous code was indeed the same code with map. Which was hacked either. It was so dumb of me to retry the code with unordered_map. I'll link my solution after the system tests. But the idea was:

          1) Find all the subsets of the songs given.

          2) Create a graph with vertices as the songs choosen and edges between them if they have a connection (common genre or song writer).

          3) If the graph has a Hamiltonian path then that order can be choosen and we try on larger size subsets.

          Update: I was able to locate it now itself: Submission

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8 months ago, # |
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Wasted a lot of time in a dsu/graph-based solution in G, when it was such a straight bitmask dp :/ pain

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8 months ago, # |
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about F, the only case answer -1 is c != a + 1 right...?

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8 months ago, # |
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How to do hacking this div-4 round?

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8 months ago, # |
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Good round, thanks mesanu SlavicG flamestorm and all testers.

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8 months ago, # |
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Is this an unrated contest? (Actually I'm just starting with CP and this is my first contest and I just checked my participated contests and it says this is an unrated contest but, here it is mentioned as rated in the description.)

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    8 months ago, # ^ |
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    no babe, rating will update after (avg 1 hours -> 1 day), you can try extension browse Carrot or CF-Predictor to get rating now

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8 months ago, # |
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Able to solve first 4 !! My greatest contest ever!!

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8 months ago, # |
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in F can anyone explain the -1 case

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    8 months ago, # ^ |
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    Initially there is one spot for a leaf. Adding type A vertex takes a spot for leaf, but creates two more (+1). Adding type B vertex takes a spot for leaf and makes one (+0). Adding type C vertex takes a spot for leaf (-1). All leaf spots should be taken so there is $$$1+a-c = 0\rightarrow a+1=c$$$.

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      8 months ago, # ^ |
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      Right. Another way to see this is — for it to be a valid tree, number of edges should be equal to the number of vertices — 1, by definition. Hence,

      2 * a + b = a + b + c — 1

      a + 1 = c

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8 months ago, # |
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Good round!

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8 months ago, # |
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Here's my live coding + commentary during the above contest: https://www.youtube.com/watch?v=kQOWiTvgah0

Would be glad if some of you find it beneficial!

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8 months ago, # |
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If I become the green man after this contest, I will dance samba!

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    8 months ago, # ^ |
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    You cant bro.Your rating is less now. Give 5 more div4.

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8 months ago, # |
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My submission 253708600 for E was hacked.

Can someone hack these as well:

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8 months ago, # |
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I solved 3 questions in this contest yesterday but there is no change in my rating. current rating -> 774

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    8 months ago, # ^ |
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    The contest is still in hacking phase. Ratings should be updated by today

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    8 months ago, # ^ |
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    It will first test the system for checking the solution of all participants.So it will take upto 24 hours after the contest timing.

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8 months ago, # |
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Why is my name not in common standings and where should I complain ??

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8 months ago, # |
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I request to disqualify the competition because they included the name "Taylor Swift" in the test cases :-)

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8 months ago, # |
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I hope testing period doesn't take so long ☺

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8 months ago, # |
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Why is the contest showing unrated for me in my profile graph? I'm still 700~ newbie

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8 months ago, # |
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i dont get any rating sir can anyone told why this happend and how to get rating

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8 months ago, # |
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Guess it's time I stop giving div4's . Div 3 and 2's go very well for me, I always get f*ed in div 4s. Don't know why.

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    8 months ago, # ^ |
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    what happened this time

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      8 months ago, # ^ |
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      Did upto D. I'm very bad at working with strings in cpp, so wasted a lot of time. Did E in Python just after it was over.

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8 months ago, # |
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Waiting for rating change be like

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8 months ago, # |
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When will the ratings come

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8 months ago, # |
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So when will the ratings be updated?

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    8 months ago, # ^ |
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    after system testing probably

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      8 months ago, # ^ |
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      On some div1/2 contests, it is as immediate as lightning once the round ends, rating updates. On other hand, for other contests, it delays. This delayed lot.

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        8 months ago, # ^ |
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        well i haven't seen it update immediately, i think it depends on hacks and new tests

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8 months ago, # |
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Happy to finally get in DIV4 rounds :), and finally promoting to pupil. The problems were nice, sadly I had not beeing able to finish all 7 as I used to much time for E. Thanks to the writers and testers for creating this beautiful round.

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8 months ago, # |
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Does code force hack by someone yesterday I successfully submit 3 questions and accept all but few moments ego it not show any questions submitted and suddenly showing only one question submitted why this happened does this happen with only everyone facing this issue and not get any rating yet

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    8 months ago, # ^ |
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    Is it hacked? Or the submission is in queue as the system testing in on.

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    8 months ago, # ^ |
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    This is normal when system testing is happening, just wait until it's over

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

So who will I see first? My children, Or The Rank??

b0f800889ebf7a8021a811250c5071da

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8 months ago, # |
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My friend solved 5 questions and green mark shows during contest but today is show again then only show 3 question are solved and submission section remaining 2 are in queue status how it possible!?

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8 months ago, # |
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i got 3 questions correct yesterday, now it is showing only that i only attempted two and one of them is still inqueue and the third one shows like i didnt even attempted it why is that?

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

I had only 5 jugs of lassi this time..... In next contest I will be mango lassi.... be tuned to see me there as well.

Yours Lovely Kesar Lassi

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8 months ago, # |
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CONGRATULATIONS the system testing took forever

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8 months ago, # |
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is this contest made unrated? Why it is showing unrated in my profile although I am having rating less than 1400

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8 months ago, # |
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When will the ratings be out

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8 months ago, # |
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when will the ranks get updated? I was 600+, solved two problems. Shouldn't my rank be changed?

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    8 months ago, # ^ |
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    Yes, your rank will be updated as soon as the new rating changes will be out. That will happen probably today.

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8 months ago, # |
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When will ratings be updated?

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8 months ago, # |
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I think there is a bug with people rating changes for this contest

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    8 months ago, # ^ |
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    oops, I think it was a lag, codeforces was showing their previous rank names even though their rating changed (for example for 1405 it was still showing pupil). But now it is ok

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8 months ago, # |
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Finally, I can change my profile picture :)

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8 months ago, # |
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Are you OK? This f**k E, I use map get TLE, but use unordered_map or add a return get AC, just because constant. This solution is like n*sqrt(n), why just give 1s?

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8 months ago, # |
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mesanu SlavicG I got a mail stating:- Your solution 253808750 for the problem 1950E significantly coincides with solutions SahooBishwajeet/253808750, candidate_3_months/253813331.

I checked the solution comparison and most of the common part was use of same #defines and variable initialization. Though the approach looks quite similar, which is to my surprise, I can assure you that I've not cheated in any form. I've followed an article on GeeksForGeeks which has a similar problem to solve the problem in the contest.

The problem approach is quite similar to the problem : https://www.geeksforgeeks.org/find-given-string-can-represented-substring-iterating-substring-n-times/

Please check the issue as I've not used any unfair means during the contest period and honestly gave the contest.

https://pasteboard.co/cdhdULYYUpns.png https://pasteboard.co/c8TFSsNWtA4b.png

Here are the comparison images for reference

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

I used Ideone in public mode as I was not aware of this and my code was copied and distributed online. I will keep this in mind from the next contest. Please do not give me any penalty. This was my first contest :(

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8 months ago, # |
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mesanu SlavicG I got a mail stating:- Attention!

Your solution 253771024 for the problem 1950D significantly coincides with solutions omkargho78/253756793, Codebind_hp/253758181, Nayan23/253767879, Fuck_BIT/253771024, BitWix/253774128, bdn_iitm/253791189, r1ashwin/253791530, onkar.jondhale/253810321.

| i have conclusive evidence that a coincidence has occurred due to the use of a common source published before the competition and i don't even know all of these, I checked the solution comparison and most of the common part was use of same #defines and variable initialization. Though the approach looks quite similar, which is to my surprise.. please recheck and do not give me penalty. mesanu SlavicG

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8 months ago, # |
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Dear Codeforces Team,

I recently received a notification regarding my solution (253819542) for the problem 1950E, indicating similarities with other solutions. Upon reviewing the mentioned solutions, I acknowledge the logical similarities. However, I would like to clarify that my approach was derived from a publicly available resource, specifically an article on GeeksforGeeks (https://www.geeksforgeeks.org/find-given-string-can-represented-substring-iterating-substring-n-times/). I further would like to clarify that I did not utilize any third-party IDE platforms like ideone, ensuring the integrity of my solution.

I understand the importance of maintaining originality and uniqueness in submissions, and I assure you that any resemblance was purely coincidental. Moving forward, I am committed to ensuring that my solutions remain distinct and reflect my own understanding and effort.

I kindly request that my solution be reconsidered in light of this explanation. I value the opportunity to participate in such amazing contests and strive to uphold the standards of integrity and authenticity.

Thank you for your attention to this matter. Please feel free to reach out if further clarification is needed.

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8 months ago, # |
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my this id and kalpesh_2495 is operated by me so unknowingly my solution where same for the contest with that id and this id.so,i am sorry for this and i will not repeat this.

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8 months ago, # |
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Hi Codeforces Team!

I have recently received a message stating that my solution to one of the problems (1950D) matches significantly with that of some other contestants. In my solution, I have checked if the factor of a number is binary decimal or not. This part of the code coincides because it is a publicly available article from where I had previously learnt it — https://www.geeksforgeeks.org/check-if-the-given-decimal-number-has-0-and-1-digits-only/

After this, if the number is a binary decimal, then we recursively call it on the number divided by the factor itself. This is a very straightforward approach and the loop will obviously run only upto sqrt(n) because we are checking for factors. So, the only place where code match can cause suspicions is due to an online resource.

I also write code on an online c++ compiler so I am not sure if it is picked up from there.

This conclusively proves that there is no code copying involved. Even though the approach is similar, the question is pretty straightforward so it must be taken into consideration that it is indeed possible. Thanks, hopefully this will get resolved and result in an unfair penalty for me

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