FelixArg's blog

Codeforces Round 955 (Div. 2, with prizes from NEAR!)
By FelixArg, history, 6 days ago, translation, In English

Hello, Codeforces!

It's been a long journey, and I'm finally pleased to invite you to participate in our Codeforces Round 955 (Div. 2, with prizes from NEAR!), which will take place on Jun/25/2024 17:35 (Moscow time).

This round will be rated for all participants with a rating is below 2100. Participants with higher ratings can participate out of competition.

During the round, you will need to solve 6 problems. You will have 2 hours to solve them.

The problems for the round were prepared by nik1998, egor4444ik, iamdimonis and me.

We sincerely thank everyone who provided invaluable assistance in preparing this round:

We are pleased to announce that NEAR has supported the round!

NEAR was founded in 2017 by Illia Polosukhin, one of the creators of Transformers, and Alex Skidanov as an attempt to build an artificial system capable of solving competitive programming problems. You can read more about that attempt here.

Ultimately, NEAR pivoted into building a blockchain protocol, which it launched in 2020.

This year, NEAR started NEAR.AI, a new lab with a mission to build AI systems that are open and available to everyone, instead of being controlled by a few mega-corporations.

One of the areas of focus is making models capable of reasoning reliably, and for that, competitive programming problems provide a great environment. To help us build it, we invite all the Russian-speaking members of the Codeforces community with a rating of Specialist or higher (1400+) to help us annotate step-by-step explanations of solutions to competitive programming problems. We want to annotate a very large set of problems of all difficulty levels, and pay relatively high rewards in NEAR per annotation. Don't speak Russian but speak English? Stay tuned here, we will be launching the same project for English-speaking people very soon, and will likely sponsor another round when it happens.

The round also features prizes in NEAR. Participants who rank in the top 16 will receive Ⓝ 16 each. The next 32 participants in the overall ranking (including unofficial participants) will receive Ⓝ 8 each. The following 64 participants will receive Ⓝ 4 each; the subsequent 128 participants will receive Ⓝ 2 each, and finally, the next 256 participants will receive Ⓝ 1 each.

Additionally, 64 random participants from the top 4096 in the overall ranking will each receive Ⓝ 4.

Score distribution: $$$500\,—\,1000\,—\,1000\,—\,1750\,—\,2500\,—\,3000$$$

Wishing everyone good luck and high ratings!

UPD: Let's continue the series of announcements with a photo of the authors :)

UPD 2: Editorial!

UPD 3: Congratulations to the winners!

Div 1:

  1. tourist

  2. jiangly

  3. SSerxhs

  4. kizen

  5. potato167

Div 2:

  1. lunchbox

  2. _JiaY19_

  3. gxy001

  4. _DongY19_

  5. Muelsyse_sep005

UPD 4: Wonderful video analysis of A-E tasks. Thank you, Shayan!

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5 days ago, # |
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Just curious, are the prizes based on full ranking or just official/Div.2 ranking only?

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    5 days ago, # ^ |
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    Prizes will be distributed among all participants of the round (including unofficial participants)

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      5 days ago, # ^ |
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      Why not just wait for a combined round instead of awarding div1 participants for solving div2 problems.

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        4 days ago, # ^ |
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        My answer
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        4 days ago, # ^ |
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        Even if div1 participants were excluded from prize list, they would have made an alt and took the prizes anyways so ig it doesn’t matter

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          4 days ago, # ^ |
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          You could just say "official participants" to reduce the number of alts but that's not my issue with this. If you're going to give prizes to div1 the contest should be at least a bit challenging for them. It's their money I guess.

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      4 days ago, # ^ |
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      That is just raw

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4 days ago, # |
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Could someone please explain what's Ⓝ in the prizes?

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4 days ago, # |
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Is anyone going to pull a rainboy?

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4 days ago, # |
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Hello everyone, hope to reach CM in this round

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4 days ago, # |
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As a tester vodacbaoan is genius

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4 days ago, # |
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December 2023: "AlphaCode 2 is worth a gazillion specialists, competitive programming is done for!"

Fast-forward to present day six months later: "Sup nerds, wanna annotate some problems?"

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4 days ago, # |
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This round will be rated for all participants with a rating of 2100 or below.

I am 2100 and I can’t register officially :(

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4 days ago, # |
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Good luck to all the participants! Codeforces Round 955 is sure to be a rewarding experience.

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4 days ago, # |
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Hey Authors

I'm curious to know all your ages/experience? since it's great to find people with 2-3 years of experience building contests!!

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    3 days ago, # ^ |
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    Hello :)

    We are 20-year-old students. Contests are done only under the dedicated guidance of our senior friends)

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3 days ago, # |
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Dashboard with 6 problems in Div-2 is a standard.

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3 days ago, # |
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As a tester problems are really interesting and challenging!

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3 days ago, # |
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As a tester, I literally tested just now because the coordinator invited me to test a few hours ago.

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I will come back after a long time not participating in contests

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3 days ago, # |
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OMG! Another FelixArg round.

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3 days ago, # |
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The expression on the face of boys explain how dangerous their mind is.

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3 days ago, # |
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What is meant by blue testing, purple testing, etc...?

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    3 days ago, # ^ |
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    they mean problem color as per rating (eg 1200,1300 green problems) if i am not wrong.

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      3 days ago, # ^ |
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      no, they "purple testing" means testers are purple. "blue testing" means testers are blue. and so on.

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3 days ago, # |
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i remember you guys removed 24-word seed restore in the default web wallet and my near coins were gone, very funny. then someone told me i have to paste it in the url to restore it. please be careful about backwards-compatibility, would hate for the new ai to suddenly vanish. looking forward to nearcrowd round 2!

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3 days ago, # |
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How score distribution is related to problem rating? I mean how do I predict what can be the difficulty of the problem?

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3 days ago, # |
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weird score distribution, I've never seen 1000 point div2c

thinking speedforce, unless upsolve D of course.

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3 days ago, # |
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Is it different from div2?

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2 days ago, # |
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64 random participants from the top 4096 in the overall ranking will each receive Ⓝ 4, that means we have a 1.5625% chances of becoming rich! All the Best!!

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2 days ago, # |
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interesting score ditribution

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it's time to be expert

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2 days ago, # |
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speedforces incoming!

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2 days ago, # |
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Can someone help me ? How am i supposed to change my compiler from c++14 to c++20 (in my own laptop) ?

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    2 days ago, # ^ |
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    You have to download and setup a newer version of GCC. Here is one of the places you can download it, under Release versions. Then you have to do the usual, adding it to system path.

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2 days ago, # |
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is there any penalty?

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33!

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2 days ago, # |
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Me after solving D : (^_^) Checking standings afterwards : (o_o)

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2 days ago, # |
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nice tasks, 2k submissions on D wow

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    2 days ago, # ^ |
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    many people solved it in very short time

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    2 days ago, # ^ |
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    thx to cheaters actually :)

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    2 days ago, # ^ |
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    I'd say it was not that hard of a problem, albeit a bit scary looking. Once you realize that you only need to get the difference of zeroes and ones in each kxk matrix to get the impact of an operation then it becomes a fairly simple number theory problem

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2 days ago, # |
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most annoying problemset ever. coupled with prizes to increase cheating.

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2 days ago, # |
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B > C

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2 days ago, # |
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Animated Video editorial for problems [A-C]:

https://youtu.be/-btzjr-u4dM

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2 days ago, # |
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problem D must have something to do with 2D prefix and maybe some gcd yes? I believe

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2 days ago, # |
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C<<<B

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    2 days ago, # ^ |
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    how to solve C? i tried queue, greedy, pretest 2 fall

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      2 days ago, # ^ |
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      actually greedy but with two-pointer
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      2 days ago, # ^ |
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      I did it with queue and greedy check my sub

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        2 days ago, # ^ |
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        yeah i find bug(like always), my solution same yours

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          2 days ago, # ^ |
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          Can you please tell me a small test case to replicate the bug for pretest 2?

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    2 days ago, # ^ |
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    indeed spent 1 hours to solve B and C under 10min

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2 days ago, # |
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meme E
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2 days ago, # |
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Can someone tell how to solve D?

My thought process for D : Find net difference between two types of mountains, this will give the value to increase. Now in each kxk submatrix, find difference between the number of two types of mountains : this will give the amount we can increase or decrease. And then I couldn't proceed further.

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    2 days ago, # ^ |
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    I wasn't able to solve it, but I think you can use prefixes on the type of mountain's matrix to optimise the calculation of which values you can increase or decrease by.

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    2 days ago, # ^ |
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    That's correct. Then you can use GCD to figure out whether you can sum the list of kxk difference into mountain_diff.

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    2 days ago, # ^ |
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    The difference should be a linear combination of the amount you can increase by each of the k*k matrices. Hence, it should be divisible by their gcd.

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    2 days ago, # ^ |
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    same, my approach was to first calculate sum of ones and zeros and get difference between them , now create prefix 2d array to store count/(difference between them) of ones and zeros in a matrix then for all possible matrix if dimension kxk save difference between one,zero in an array so now my problem boils down to classic Coin Change DP(with infinite coins of each arr[i]),but it takes O(sizeofArray*Sum) probably TLE

    any hints please ??

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    2 days ago, # ^ |
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    Suppose the difference of the numbers of each type of mountain is $$$k_1,\dots,k_\ell$$$. Recall from number theory that the only numbers we can form as a $$$\mathbb{Z}$$$-linear combination of integers $$$k_1,\dots,k_\ell$$$ (i.e. $$$c_1k_1+\cdots+c_\ell k_\ell$$$ for integers $$$c_i$$$) are multiples of $$$\gcd(k_1,\dots,k_\ell)$$$. Therefore, if the difference is some multiple of the $$$\gcd$$$ of all the $$$k_1,\dots,k_\ell$$$, then there is some series of moves we can do to get the difference to zero (plus some edge cases where some/all of the $$$k_i$$$'s are zero).

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      2 days ago, # ^ |
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      I was knowing ax + by = c has solutions only if c is divisible by gcd(a,b) but was not knowing this is true for any number of variables.

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        2 days ago, # ^ |
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        If you have $$$a_1x_1+a_2x_2=c$$$ iff $$$\gcd(a_1,a_2) \mid c$$$, then you can write (for some $$$v$$$)

        $$$ a_1x_2 + a_2x_2 + a_3x_3 = (a_1x_2 + a_2x_2) + a_3x_3 = v\gcd(a_1,a_2) + a_3x_3 = d $$$
        $$$ \iff \gcd(\gcd(a_1,a_2),a_3) = \gcd(a_1,a_2,a_3) \mid d, $$$

        and you can continue by induction :)

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          2 days ago, # ^ |
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          I didn't know this. If it's so simple to know if a sum can be obtained by some combination, then why don't we use this property in subset sum problem or coin change problem?

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            2 days ago, # ^ |
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            It's simple because every item can be used for any integer number of times, but it's not the case for subset sum problem.

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              2 days ago, # ^ |
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              This will only hold when the coefficients can be any integer (including negative) ?

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                2 days ago, # ^ |
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                Yes. For example, with $$$2,3$$$ you can form $$$1$$$ with negative coefficients as $$$1(3)+(-1)2$$$ but clearly not with positive (the result would be greater or equal to 2!)

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                2 days ago, # ^ |
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                It would hold if the coefficients can be any integer. But the reverse direction may not be true (i.e. it's possible to get all multiple of $$$gcd(a_1, a_2, \ldots, a_n)$$$ even if some of the value can't be used for any integer number of times).

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      2 days ago, # ^ |
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      Is there a name for that theorem or is it just a common concept?

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    2 days ago, # ^ |
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    You can find the net difference in all $$$K \times K$$$ submatrices and let gcd of these differences be $$$G$$$. It can be proven that you can only change the total height difference by some multiple of $$$G$$$, therefore you only need to check if the height difference is divisible by $$$G$$$.

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2 days ago, # |
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How to do B?? Spent more than an hour and couldn't solve it.

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    2 days ago, # ^ |
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    After a while, the value of x just starts to loop in between 1 and y-1 . So just find out the position at the end making use of some %

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      2 days ago, # ^ |
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      Tried something similar to that.

      I tried to convert x to 1. If it was possible then I gave x + k%(y-1) as answer and if not then returned x only.

      Submission : 267370934

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        2 days ago, # ^ |
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        At First , we need to ensure that x<y ,only after that x can become 1.

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2 days ago, # |
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So you've really decided to use the $$$3 \cdot n + 1$$$ problem in a contest? I'm really salty right now if you didn't notice.

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2 days ago, # |
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Unable to pass the second problem

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2 days ago, # |
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even though i couldn't solve it, i feel that C is much easier than B. From the One's I've read, A-C. I can say that the problems were very interesting and I think i would enjoying upsolving them.

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    2 days ago, # ^ |
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    The trick to B is to try to simulate the operations and output the x values as it changes, then it becomes apparent that it starts looping between 1 and y-1 when x reaches a value equal to y or less. And to get to that point you just perform as many operations as you need to get x to the next multiple of y but you don't actually need to add 1 every time just find the next multiple of y and do all the ops at once

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I thought in A for almost 20 minutes but then I activated guessing mode and was able to figure it out in less than 10 second

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B was very hard for people who aren't that good at math. :(

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    2 days ago, # ^ |
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    u basically simulate the process, don't need to be at math

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    2 days ago, # ^ |
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    i thought it only needed memoization. but there's a math trick here?

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      2 days ago, # ^ |
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      yeah you can basically simulate it until x < y, and in that case x will go up to y and then down to 1 in a loop forever.

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Is C possible with monotonic queue DP that define f[i] as the answer range 1 to i? It seems that it only need a simply greedy.

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Why are all the A's getting hacked..?

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Thanks for the contest. I got my lesson. I wont give any contest from now on which has the word TrAkToR in its description.

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2 days ago, # |
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Why so many hacks on A? (observing those on page 1 of standings)

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2 days ago, # |
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A: If the sign of (x-y) changed, then (x-y) must be 0 at some time, so answer is NO. Otherwise, the answer is YES.

B: If x<y, then the value of x will change among {1, 2, ..., y-1} periodically, so the answer will depend on k%(y-1). Otherwise, after (y-x%y) operations x will become (x+y-x%y)/y, which is not greater than (x/y)+1. Because y>=2 this can happen not more than log(x) times until x<y, so we can simulate the process in O(log(x)).

C: Let dp[i] be the anwser for first to i-th cards. We can binary search for the largest j such that sum(j+1, i)>=l, if sum(j+1, i)<=r we have dp[i]=max(dp[i-1], dp[j]+1), otherwise dp[i]=dp[i-1].

D: Let the balance of heights B=(sum of snowy heights)-(sum of not-snowy heights). If B==0 initially the answer is YES. If we let c[i][j]=1 when (i, j) is snowy, c[i][j]=-1 otherwise, then when we do operation on a k*k square, the change of balance will be sum(i,j)(c[i][j]) where (i,j) is in the square. So we can calculate the sum of c[i][j] over all k*k submatrices using 2D prefix-sum, and by the bezout's theorem, the change of balance can be any multiple of GCD of these sums. In order to make B=0, the initial value of B must be the multiple of GCD.

E: First think about case for n=2**d. If k>=d the answer is n*(n+1)/2, and if k==d-1 the answer is (n-1)*n/2, let's assume k<=d-2. We can see the answer is sum(1<=r<=k+1)((2**r-1)*(2**r)/2 * C(d-r-1,k+1-r)). Denote this as f(d, k).

Proof

Now think about arbitrary n. We can split [0, n) into several intervals with length of 2**j, where n has the j-th bit. If we let L=floor(n/2**(j+1))*(2**(j+1)), R=L+2**j, the answer on interval [L, R) will be f(j, k-popcount(L)). If popcount(R-1)>k, there will be no valid interval containing both R-1 and R, so we can add this to the answer and consider for [R, n) seperately. Otherwise, when popcount(R-1)<=k, we can see that for any R<=x<n we have popcount(x)<=k, so we can add (n-L)*(n-L+1)/2 to the answer.

F: Let L be the minimum i such that a[i]>a[i+1], R be the maximum i such that a[i-1]>a[i]. Then at least we need to sort subarray [L, R]. Let X=min(L<=i<=R)(a[i]), Y=max(L<=i<=R)(a[i]), if there are any L1<L such that a[L1]>X or any R1>R such that a[R1]<Y, then we also need to sort them. To find L,R we can maintain a set S={1<=i<n: a[i]>a[i-1]} which can be modified in O(log(n)) each operation. To find X,Y we can use a segment tree, and to find L1, R1 we can use binary search on a[i].

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    2 days ago, # ^ |
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    how to solve F?

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    2 days ago, # ^ |
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    C is greedy $$$O(n)$$$, you don't need a binary search.

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      2 days ago, # ^ |
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      yes but bs solution is easier to understand why it works.

      also easy to implement without considering different cases. check his solution 267340021

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        2 days ago, # ^ |
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        The greedy isn't too hard to prove either. At any step, you chose a valid interval with minimum right side index. That is correct because you end up with more possibilities for the rest of the problem than with any other valid leftmost interval (since its right index would be larger).

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2 days ago, # |
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Who can prove D?

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    2 days ago, # ^ |
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    Bézout's identity

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      2 days ago, # ^ |
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      Wow, first time hearing this. Thanks! Very counterintuitive for me.

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    2 days ago, # ^ |
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    I just thought about Linear Diophantine equation, and the condition of it to be solvable is in CP-algorithm :v.

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      2 days ago, # ^ |
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      I am just not studying number theory, haha. I just thought about knapsack.

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    2 days ago, # ^ |
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    If d[i][j] is the positive difference between the number of snowy caps and bare caps in the k*k submatrix whose top left corner is (i,j), then you can change the total difference in sum of heights, by c * gcd(d[i][j]), where c is any integer. (bezouts identity).

    So just compute this gcd and check if it divides the positive difference between the sum of snowy heights and the sum of bare heights.

    Can obviously be done in linear time by precomputing the rectangles.

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2 days ago, # |
← Rev. 2   Vote: I like it +6 Vote: I do not like it

Wrote a overcomplicated DP + Segment Tree solution for C. D was a good problem.

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2 days ago, # |
  Vote: I like it +5 Vote: I do not like it

In fact, I think that question C is much less difficult than question B, and I was thwarted by question B for forty minutes but passed C in ten minutes.

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2 days ago, # |
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D was a great problem, shame I couldn't code it up in time. Need to speed up.

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2 days ago, # |
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Wow , first time got TLE just by using a % operatoion,

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2 days ago, # |
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int solve(int i,auto& psum,int* dp,int n,int l,int r){
    if(i>=n) return 0;
    if(dp[i]>=0) return dp[i];
    int take=psum[i]+l;
    int nextpos=lower_bound(psum.begin()+i,psum.end(),take)-psum.begin();
    int val = 1;
    if (nextpos >= n || (nextpos < n && psum[nextpos] > (psum[i]+r)))
        val = 0;
    return dp[i]=max(val+solve(nextpos+1,psum,dp,n,l,r),solve(i+1,psum,dp,n,l,r));
}


void helper(){
    int n,l,r;
    cin>>n>>l>>r;
    vi a(n);scan(a);
    vector<int> pxsum(n);
    pxsum[0]=a[0];
    for(int i=1;i<n;i++){
        pxsum[i]=pxsum[i-1]+a[i];
    }

    int dp[n];
    fill(dp,dp+n,-1);
    cout<<solve(0,pxsum,dp,n,l,r);
    
}

what is wrong in the above code for c? Wtf is wrong...

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2 days ago, # |
  Vote: I like it 0 Vote: I do not like it

Can anyone teach me how to past B problem?

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

    When $$$x$$$ will become less than $$$y$$$, there will be a cycle: $$$x$$$, $$$x+1$$$, ..., $$$y-1$$$, $$$1$$$, $$$2$$$, ..., $$$x-1$$$, $$$x$$$, which cost $$$y-1$$$ moves, so you just take $$$k$$$ modulo $$$y-1$$$ after $$$x$$$ will become less than $$$y$$$.

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2 days ago, # |
  Vote: I like it +16 Vote: I do not like it

Lol, i just read F and isn't it just much much easier, than D and E? It took me like five minutes to come up with the solution.

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    43 hours ago, # ^ |
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    lol it appears to me that it's harder than D and E, im just being stupid...

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2 days ago, # |
← Rev. 3   Vote: I like it -16 Vote: I do not like it

Can anybody please help me understand the problem with my code for problem C(Boring days) ~~~~~

signed main() { ll t; cin >> t; while(t--) { ll n,l,r; cin >> n >> l >> r;

vector<ll> a;
  for(ll i=0;i<n;i++) {
    ll x;
    cin >> x;
    a.push_back(x);
  }

  ll i = 0;
  ll j = 0;
  ll sum = 0;
  ll count = 0;

  while(j<n){
    sum+=a[j];
    if(sum<l){
      j++;
    }
    else if(sum<=r and sum>=l){
      count+=1;
      i = j+1;
      j++;
      sum = 0;
    }
    else if(sum>r){
      while(sum>r){
        sum-=a[i];
        i++;
      }
      if(sum<=r and sum>=l){
        count+=1;
        i = j+1;
        j++;
        sum = 0;
      }
      else{
        i = j+1;
        j++;
        sum = 0;
      }
    }


  }

  cout << count << endl;

}

}

~~~~~

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    2 days ago, # ^ |
      Vote: I like it -25 Vote: I do not like it

    bro, your final else block is the problem, its the same as if block

    include <bits/stdc++.h>

    using namespace std;

    typedef long long ll;

    signed main() { ll t; cin >> t; while(t--) { ll n,l,r; cin >> n >> l >> r;

    vector a; for(ll i=0;i<n;i++) { ll x; cin >> x; a.push_back(x); }

    ll i = 0; ll j = 0; ll sum = 0; ll count = 0;

    while(j<n){ sum+=a[j]; if(sum<l){ j++; } else if(sum<=r and sum>=l){ count+=1; i = j+1; j++; sum = 0; } else if(sum>r){ while(sum>r){ sum-=a[i]; i++; } if(sum<=r and sum>=l){ count+=1; i = j+1; j++; sum = 0; } else{ j++; //only change } }

    }

    cout << count << endl; }

    }

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      2 days ago, # ^ |
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      267411889 check this, cuz the copy paste was horrible

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        2 days ago, # ^ |
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        Thanks , but can you explain why that was wrong in which test case it will fail

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          2 days ago, # ^ |
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          well you else block is basically (sum < l) right? and in that condition you need to j++, i.e., only move the front pointer as the sum is already smaller than l.

          For example, 6 9 10 1 3 3 5 4 8 the original code will fail this test case and give 0, instead of 1. because it will not go to the pair i=5,j=4(which sums up to l) but would rather go to i=4,j=4

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2 days ago, # |
  Vote: I like it 0 Vote: I do not like it

267409135 Can anyone tell me why is this not working? Like is my logic completely wrong or is there some edge case that the code misses?

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2 days ago, # |
← Rev. 3   Vote: I like it -6 Vote: I do not like it

My submission Can anyone pls tell me why this fails.... It shows wrong ans on 4288 no of test case 2

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2 days ago, # |
  Vote: I like it -6 Vote: I do not like it

267392213 Can anyone pls tell me why this fails.... It shows wrong ans on 4288 no of test case 2

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    2 days ago, # ^ |
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    the way you update j may be the mistake. I have solved using same approach (link)

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      2 days ago, # ^ |
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      I have updated j=I+1 in same iteration You updated j=I in next iteration..... I think both are same thing as in next iteration I is incremented by 1...then why my soln fail???

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        2 days ago, # ^ |
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        while (i < j) was a mistake it shd be while (i <= j)

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          2 days ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          267425411 using i==j giving error on 910 case of test case 2

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            2 days ago, # ^ |
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            you logic in while loop was the mistake, i have updated with slight modification in your code which passed link

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              2 days ago, # ^ |
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              thank you bro finally after 2 hrs i got my mistake thanks for helping out

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2 days ago, # |
← Rev. 2   Vote: I like it +7 Vote: I do not like it

Hello everyone, I need somebody to hack my friend's code: https://mirror.codeforces.com/contest/1982/submission/267413232

I am so sure that His code will overflow since he just used 32bits integer (int / i32). Hurry up! Don't let him discover it! Just hack the submission!

PS: His computer's battery has run out and I submitted his code after contest. But why it AC!!!!! I cannot accept it!!!!!

Thanks!

Update:

Sorry, his code is correct. Just sum up all the "-1" and "+1" will not exceed i32. My bad! Actually, he cannot submit his code since the battery problem, I want to easy him that his code maybe cannot pass the test (So that he will not lose this big chance to increase his rating). Okay...... Now he will be definitely very sad and regretful.

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    2 days ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    Hacking phase was open during contest. Its over now

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

    really? I thought $$$500^2\times 48$$$ won't overflow! did I miss something?

    upd: he even changed the char '1' into int 1! it seems more unlikely to overflow now.

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2 days ago, # |
← Rev. 2   Vote: I like it +39 Vote: I do not like it

Is filling "TON wallet" field (and "Secret Code") with account id from mynearwallet in settings enough for getting reward in NEAR cryptocurrency for current round? Or what should I do to get it?

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2 days ago, # |
  Vote: I like it 0 Vote: I do not like it

I did surprisingly well in this round. This was a really fun one, questions were not very straightforward, I liked it a lot.

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2 days ago, # |
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Why is this contest unrated for me? Any reason codeforces for this action

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2 days ago, # |
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In c, how it is sure that x will become 1 in less than or equal to k operations?

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

    lets say x=16 y=3 k=1, clearly we are left with x=17

    lets say x=16 y=3 k=3, 16->17->2->1, and x became <y

    the idea is: after every deletion x decreases from what it was originally even if it increased a little before getting to a multiple of y. and if x always decreases then after some time it must go to 1, after that it loops from 1 to y.

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2 days ago, # |
  Vote: I like it +1 Vote: I do not like it

My rating is 1110.I participated in this contest. But my rating not changed.Why?

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    2 days ago, # ^ |
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    same i don't know why its not reflecting

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    2 days ago, # ^ |
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    I hope that the rating will be updated after some time. Many people have this nuisance

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2 days ago, # |
← Rev. 2   Vote: I like it -14 Vote: I do not like it

Thank you

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2 days ago, # |
  Vote: I like it 0 Vote: I do not like it

Till now i was happy that i became specialist but all of a sudden ratings changes is disappeared, can i know what's the reason behind it?

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    2 days ago, # ^ |
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    They are probably running plagiarism checks and they will need to recalculate the ratings so it's kind of messy right now

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      2 days ago, # ^ |
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      But why they released earlier and now making such messy but i don't think that this happened before

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        2 days ago, # ^ |
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        This happens sometimes but usually the plagiarism checks are ran for multiple contests at once (And then you have the message that "the ratings for a few contests are temporarily rolled back") but I assume they wanted to do a single contest plagiarism check here because of the contest having prizes

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          2 days ago, # ^ |
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          In that case then they have check plagiarism for both today's contest and recent div 3 contest.

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            2 days ago, # ^ |
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            Maybe, not sure how exactly it works but I'm assuming that the plagiarism check has to be ran manually and now they are doing it for this contest because of the prizes

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2 days ago, # |
← Rev. 2   Vote: I like it -23 Vote: I do not like it

Subject: Issue with Judgement in Problem C

Hi,

I encountered a possible error in the judgement of my submission for Problem C during Codeforces Round 955 (Div. 2). It seems there was a bug during the judging process.

Details:

My Submission During Contest: Submission 267358679 My Submission After Contest: Submission 267429867 During the contest, in the 2nd pretest, 6th test case, my submission was judged incorrectly. The actual output should be 0, but it was judged as 1, resulting in a wrong submission.

After the contest, I resubmitted the same solution, and it was judged correctly with the output being 0.

I would appreciate it if the issue could be reviewed and corrected as it affected my contest standing unfairly.

Thank you for your attention.

Best regards, Deshik conan45

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    46 hours ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    Bruh, it's not even the same code, I don't know what you are trying to achieve

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46 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

I am awaiting to be in random 64 contestants!!!

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45 hours ago, # |
  Vote: I like it +2 Vote: I do not like it

I have one question for the cf community, why to give the score distribution beforehand, people can initially guess the difficulty of the contest, isn't it non exciting it would be more fun if we are not known that and may be we can keep it constant in every contest

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38 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

https://mirror.codeforces.com/blog/entry/130861 Help me guys

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38 hours ago, # |
  Vote: I like it +4 Vote: I do not like it

FelixArg , My solution for D shouldn't have passed the test cases, but it did.

  • I knew the current difference between type1Sum, and type0Sum. ( let's call it RequiredDifference )

  • I knew for each of the sub-rectangle of size k*k, what are achievable differences. Lets call them (d1, d2, d3 ... dk).

I knew that I had to do a1 * d1 + a2 * d2 + a3 * d3 + ... ak * dk = RequiredDifference

I didn't know how that gcd ( d1 , d2 ... dk ) | requiredDiff .

But I knew, for ax + by = z , then gcd(x, y) | z. So I used this two variable equation property and passed test cases. Refer to my solution.

The test cases were weak for D. Ideally my solution shouldn't pass. It was later hacked by djm03178 .

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    34 hours ago, # ^ |
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    bro i wrote the same solution as yours and mine was also hacked lmao.

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37 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

So many people here are saying that B was a lot difficult than C. I did B in 20 mins, but it took me almost a day and more than 10 attempts to finally get AC on C. I had actually figured it in some time but it was giving TLE again and again. Don't know if I am too dumb or too smart lol!!

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

Exposing Cheating in Competitive Programming Cheating undermines the spirit of competitive programming. The Telegram channel selling Codeforces solutions, with Gautam_Himanshu implicated in using these solutions to unfairly boost their ranking. This not only disadvantages honest participants but also erodes the platform's credibility. The community must stay vigilant, report suspicious activities, and promote ethical practices. By addressing such issues, we can ensure a fair and rewarding environment for all. Let's work together to uphold the values of integrity and fair play in competitive programming. Report any evidence of cheating to maintain our community's standards.

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34 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Creating: .cfuser.near.

Transaction to create the account was sent, but the account is not on chain yet. Please refresh the page. If the problem persists, please reach out to the admins

Can you please help me?

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33 hours ago, # |
  Vote: I like it +3 Vote: I do not like it

Even though I ate negative delta the problems were very good, I enjoyed the round so much.

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32 hours ago, # |
  Vote: I like it +1 Vote: I do not like it

Hey folks! Here's an attempt of an explanation of problems A to D of this context with Tourist's codes https://www.youtube.com/watch?v=3fPvRAowuqU. Would be glad if you find it informative!

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29 hours ago, # |
  Vote: I like it +16 Vote: I do not like it

How do I redeem the 1N prize? Sorry, this is the first time I've won something in an online contest, so I don't know how to redeem stuff. Thanks in advance for the reply!

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28 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Hey, I have just received a message that my solution 267390457 for the problem 1982C significantly coincides with some other people. I don't use any online source and don't even know the persons with whom my code coincides. This happened because the problem had a simple code of 6 7 lines, so it was a coincidence that someone wrote something similar to what I did. Even if you go through a number of other submissions, the logic is nearly the same, and since it is a small code, it matches several people. Also, I received this message for one question only, but all other questions I answered were skipped.

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27 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Dear Codeforces Admins,

I recently received a notification regarding a significant coincidence between my solution (267386858) for problem 1982B and those of Muhammad_Tawfiq (267384934) and Antartic_Nafeez (267386858). I want to clarify that my solution was independently developed, and I have not shared my code with anyone else.

However, I understand that such coincidences can occur, especially if there are common approaches or algorithms widely known and used among participants. I assure you that I did not use any external sources that would violate the competition rules. If there is any specific evidence or further details needed from my side, please let me know, and I will be happy to cooperate fully.

Additionally, I do not use public repositories or platforms like ideone.com with default settings, which could unintentionally expose my code. If there are any recommendations on how to further ensure the integrity of my solutions and avoid such coincidences in the future, I am open to suggestions.

Thank you for your understanding and consideration.

Best regards, [Antartic_Nafeez]

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27 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Dear Codeforces Admins, I received a notification regarding a significant coincidence between my solution (267366526) for problem 1982C and those of Mazen_eldar/267341490 and jainlakshit/267354671. I want to clarify that my solution was independently developed, and I have not shared my code with anyone else. And i don't even know who they are and how it happened but i think such coincidence can occur in this problem because the solution was so trivial that anyone would have thought of the same solution and it can happen that someone else has written same code as me but it is surely a coincidence i can guarantee you i am loyal with my codeforces handle and i would love if my rating increases so i will not risk my handle by doing such pitty things, please consider my request i am genuinely saying it was just an coincidence and i will try that it never happens again. So please do not skip this contest and i expect my justice from you guys.

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27 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Dear Codeforces Team,

I recently received a notification regarding a significant coincidence between my solution for problem 1982B (solution ID: 267384934) and the solutions submitted by Muhammad_Tawfiq (solution ID: 267384934) and Antartic_Nafeez (solution ID: 267386858). I take such matters very seriously and would like to address this situation promptly.

Firstly, I would like to clarify that my intention has always been to adhere to the rules and maintain the integrity of the competition. I assure you that I have not intentionally shared my code with anyone nor copied from other contestants.

To provide more context:

  1. Code Review: I have thoroughly reviewed my code and confirmed that it is my own work. However, I understand that similar solutions can sometimes arise due to common approaches to solving the problem.

  2. Use of Public Platforms: I have not used public platforms like ideone.com with default (public) settings to share my code. All my code is developed locally or on private, secure platforms.

  3. Potential Common Source: If there was any unintentional similarity due to a common source or widely known algorithm published before the competition, I am more than willing to provide details of my references and thought process.

I request a thorough review of this case and am ready to cooperate fully by providing any additional information or evidence required. My goal is to clear up any misunderstandings and continue participating in Codeforces competitions fairly.

Thank you for your attention to this matter.

Best regards,

[Muhammad_Tawfiq]

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    20 hours ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Almost the same code,stop joking bro.

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      9 hours ago, # ^ |
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      exactly the same code, he's definitely joking

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27 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Hello Codeforces,This is in regard to plagiarism being put on one of my solutions. I want assure you that no such incident has been done by me the only common issue can be a code template i use for some questions .You can check all my previous contests and submissions.This was the first time i used this template seeing many other use a template of their own.If unintenionally I have made a mistake I apologize and assure that i wont be using templates as well from now on

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27 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

I recently received a notification regarding a significant coincidence between my solution for problem 1982B (solution https://mirror.codeforces.com/contest/1982/submission/267370683) and the solutions submitted by Om3466 (https://mirror.codeforces.com/contest/1982/submission/267357286 ) . I take such matters very seriously and would like to address this situation promptly.

Firstly, I would like to clarify that my intention has always been to adhere to the rules and maintain the integrity of the competition. I am confident that I have not shared my code with anyone nor copied from other contestants.

To provide more context:

Code Review: I have thoroughly reviewed my code and confirmed that it is my own work. However, I understand that similar solutions can sometimes arise due to common approaches to solving the problem.

Use of Public Platforms: I have not used public platforms like ideone.com with default (public) settings to share my code. All my code is developed locally or on private, secure platforms.

I request a thorough review of this case and am ready to cooperate fully by providing any additional information or evidence required. My goal is to clear up any misunderstandings and continue participating in Codeforces competitions fairly

Please see to it ASAP and reply .

Thank You

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

Hello MikeMirzayanov, I got coincidence message on Round 955 div2 on Problem B. My solution is 267357971. It was a pure Coincidence because the logic of code was not so big and also , I submitted the code, a lot earlier than other user. Neither I have use public sites like ideone.com. All my codes are developed and run locally on my system. Please do the needful.

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26 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Dear Codeforces Admins,

I recently got a message that my 1982C solution, sriragt/267362951, significantly coincides with several other people's solutions. However, I believe my solution to this problem was a relatively short and straightforward two pointers approach that was replicated by many other submissions as well. I do not know any of these other people, and I believe that the similarity of our solutions is simply a coincidence because my solution is only 10 lines and there is not that many different ways to actually complete this solution. All of my code that I have written was created 100% by me, so I do not understand why my submission was flagged. Please let me know if you would like more information from me.

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26 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

My solution for B has been flagged as being plagiarised. However, I can assert that it was not copied and I didn't partake in any other such activities. My performance in this contest was on-par/worse than normal. I understand why the error may have occurred as many other participants similarly identified the algorithm. Please do the needful.

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26 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Dear Codeforces Team, I got a notification that my solution (267400970) for the problem 1982B coincides with that of many other people. This is the first time I've gotten that kind of message, and I don't have any idea how it coincides. Also, all my submissions in that contest were skipped. What should I do now? Can anyone help me?

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25 hours ago, # |
  Vote: I like it 0 Vote: I do not like it

Hey Both of Specialspecies and ThisTomCantTinker are my accounts. I submitted a solution on SpecialSpecies account to see if there will be a -50, it got accepted so I submitted it on my actual account(ThisTomCantTinker). I was not aware that they would flag it like that TT. I emailed back too to the mail by codeforces with screenshots of score boards for this Div2, from a login from both my accounts.

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    13 hours ago, # ^ |
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    I am sorry I did not read that alternate accounts are not allowed, am fairly new to the process

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25 hours ago, # |
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Dear codeforces admins, Recently i got a message from system saying that my problem B submission(267368310) is coinciding with some users. I solved the question on my own and I want the system to consider my submission. I do not know anything about the coinciding submissions, I write my code un my own created template which is consistent through many submissions. Please help!!

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20 hours ago, # |
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Hello Codeforces Team,

Today I received this message,

Attention!

Your solution 267373789 for the problem 1982C significantly coincides with solutions tikam/267373789, heyPaarth/267384529. 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). If you have conclusive evidence that a coincidence has occurred due to the use of a common source published before the competition, write a comment to post about the round with all the details. More information can be found at http://mirror.codeforces.com/blog/entry/8790. Such violation of the rules may be the reason for blocking your account or other penalties. In case of repeated violations, your account may be blocked.

Here is my solution link: https://mirror.codeforces.com/contest/1982/submission/267373789

I solved the question using the simple sliding window approach and used local IDE as well. I just think that the approach is too obvious to anyone as many of the sliding window questions have same approach if someone understands the logic. After receiving the email, I was pretty much sure that such an approach will be found on Internet, so I searched and found the same sliding window approach for a question in a GFG article. Here is the link: https://www.geeksforgeeks.org/find-subarray-with-given-sum/

I also had 3 wrong submissions during the contest for the same question whose links are given below: https://mirror.codeforces.com/contest/1982/submission/267372602 https://mirror.codeforces.com/contest/1982/submission/267369000 https://mirror.codeforces.com/contest/1982/submission/267367808

in these also I was using same sliding window approach, but was overcomplicating the things to miss some edge cases. So I simplified it in the later one.

I also even don't know about the other person that who is he. Here is his solution that coincides with me: https://mirror.codeforces.com/contest/1982/submission/267384529

I also submitted the solution early than him, so there is no chance that I cheated the solution.

You can check all the submissions and article shared along with your team. I Would be eager to hear back from you about further proceedings and reply.

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17 hours ago, # |
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Hello I am trying to solve problem D but my submission:https://mirror.codeforces.com/contest/1982/submission/267596120

is failing for the following test case:

3 4 3

46 49 50 1

19 30 23 12

30 25 1 46

1000 0100 0010

Please let me know if y'all can help me correct my code Also I read the editorial and tried to recreate the steps but I am unable to understand the why we consider the differences in ones and zeros, if anybody could explain the intuition behind the why that would be great

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16 hours ago, # |
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hi everyone, can you tell the issue in my submission for problem C https://mirror.codeforces.com/contest/1982/submission/267600026

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15 hours ago, # |
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I am writing to express my concern regarding the recent cancellation of my contest submission due to the coincidental similarity of one of my Three codes with another participant's code. While I understand the importance of maintaining the integrity of the contest, I would like to clarify that this similarity was purely coincidental and not a result of any form of misconduct.

It is not uncommon for similar solutions to arise independently, especially when addressing common problems or using standard approaches. I assure you that all my submissions were created independently and with the utmost integrity. The code I made had a simple logic of checking next divisor on less than, equals to and greater than; if the next divisor is less than k , then u can just return x+k if its equals to k then divide x until if indivisible and if it's greater than k u can use similar double while loop to check the desired number.

I have submitted all the code by writing them manually on VS code first then submitting it. I don't use libraries or any other stuff that must be one of the reason which might have gotten my code to be similar to others, but in honor regards I want u to consider if my only 1 out of 3 code is found similar but I made Problem C which is harder than B myself. Which clearly shows that I have enough potential to solve B too on my own.

I kindly request a reconsideration of my disqualification, taking into account the originality and uniqueness of my other two codes. I am more than willing to provide any additional information or clarification needed to resolve this matter fairly.

Thank you for your attention to this issue. I look forward to a fair resolution.

Sincerely

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13 hours ago, # |
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Dear Codeforces Admins,

I recently received a notification regarding a significant coincidence between my solution (267385864) for problem 1982B and those of a few other contestants. I want to clarify that my solution was independently developed, and I have not shared my code with anyone else.

However, I understand that such coincidences can occur, especially if there are common approaches or algorithms widely known and used among participants. Since the program is a bit tricky with simple approaches and gives a TLE, it might be possible that the other participants might have used the same technique as I did since there are only a handful of ways to solve to the knowledge that I have. I assure you that I did not use any external sources that would violate the competition rules. If there is any specific evidence or further details needed from my side, please let me know, and I will be happy to cooperate fully.

Additionally, I do not use public repositories or platforms like ideone.com with default settings, which could unintentionally expose my code. If there are any recommendations on how to further ensure the integrity of my solutions and avoid such coincidences in the future, I am open to suggestions.

Thank you for your understanding and consideration. kv_2205

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11 hours ago, # |
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Dear Codeforces Admins,

I recently received a notification regarding a significant coincidence between my solution (2267381255) for problem 1982B and those of a few other contestants. I want to clarify that my solution was independently developed, and I have not shared my code with anyone else.

However, I understand that such coincidences can occur, especially if there are common approaches or algorithms widely known and used among participants. Since the program is a bit tricky with simple approaches and gives a TLE, it might be possible that the other participants might have used the same technique as I did since there are only a handful of ways to solve to the knowledge that I have. I assure you that I did not use any external sources that would violate the competition rules. If there is any specific evidence or further details needed from my side, please let me know, and I will be happy to cooperate fully.

Additionally, I do not use public repositories or platforms like ideone.com with default settings, which could unintentionally expose my code. If there are any recommendations on how to further ensure the integrity of my solutions and avoid such coincidences in the future, I am open to suggestions.

Thank you for your understanding and consideration. mohantyanurag2004

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9 hours ago, # |
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Dear Codeforces Team,

I hope this message finds you well. I recently received a notification regarding a significant similarity between my solution for problem 1982B (submission 267373729) and the solution submitted by pyramid7 (267361572). I take this matter very seriously and wish to address it promptly and transparently.

First and foremost, I would like to emphasize my commitment to the rules and integrity of the competition. I assure you that I have neither shared my code with anyone nor copied from any other contestant.

To provide additional context:

  1. Code Review: I have meticulously reviewed my code and can confirm that it is entirely my own work. I understand that similar solutions can occasionally occur due to common methodologies in problem-solving.

  2. Use of Public Platforms: I have not utilized public platforms such as ideone.com with default (public) settings to share my code. All my development is conducted locally or on secure, private platforms.

I kindly request a comprehensive review of this case and am more than willing to provide any additional information or evidence required to clear up any misunderstandings. My objective is to ensure fair play and to continue participating in Codeforces competitions with integrity.

Thank you for your attention to this matter. I look forward to your prompt response.

Best regards,
Dhanush B

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9 hours ago, # |
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seems everyone has received plagiarism message

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    9 hours ago, # ^ |
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    is that the reason why rating change was rolled back?

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      9 hours ago, # ^ |
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      is rating rolled back for everyone?

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        9 hours ago, # ^ |
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        yeah, i didn't get the plagiarism message but my rating is also rolled back.

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          9 hours ago, # ^ |
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          same, i also didn't get plagiarism message but my rating is rolled back

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            9 hours ago, # ^ |
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            I believe it will come back as maybe they are checking more cheaters and whoever didn't cheated they will get there rating back

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    9 hours ago, # ^ |
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    seems like alot of cheaters got exposed in this round great thing tbh

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    7 hours ago, # ^ |
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    Cheaters are even copying their excuse messages. Kind of funny.

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8 hours ago, # |
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How long do the rollbacks take? I need to submit my resume with a screenshot of rating as proof in 4 hours:(

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6 hours ago, # |
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Hello everyone! I am new here, so I don't know how to correctly post (yet)! I was trying C for hours on end, and somehow it fails the ~4kth test case in test case 2 :( Can someone please help me find the bug I'm doing? P.S I'm doing a greedy strategy: https://mirror.codeforces.com/contest/1982/submission/267702193

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