Stefan2417's blog

Codeforces Round #948 (Div. 2)
By Stefan2417, 4 weeks ago, translation, In English

Hello, Codeforces!

After a year of anticipation and several complete changes to the problem set, we are glad to invite you to take part in Codeforces Round 948 (Div. 2), which will start on Sunday, May/26/2024 17:35 (Moscow time). This round will be rated for participants whose rating is below 2100. Participants with higher ratings may participate out of the competition.

You will be given 5 problems and 2 hours to solve them. All the problems in the round are created and prepared by Stefan2417 and alexchist.

The round may include one or more interactive problems. Please read this blog to get familiar with this type of problems.

We would like to thank:

  • Vladithur for excellent round coordination!

Score distribution: $$$500 — 1250 — 1750 — 2000 — 2500$$$.

Your perception of the scoring may vary, so be sure to read the later problems if you get stuck on one.

Upd: Congratulations to the Winners!

Div 2:

  1. sun_gan_chou_yu_guan

  2. Maksiwelle

  3. suomynonA

  4. new_mistakes

  5. Kosyaaa

Div 1+2:

  1. tourist

  2. Sugar_fan

  3. sun_gan_chou_yu_guan

  4. abc864197532

  5. BurnedChicken

Upd: editorial

  • Vote: I like it
  • +273
  • Vote: I do not like it

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

Auto comment: topic has been updated by Stefan2417 (previous revision, new revision, compare).

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3 weeks ago, # |
  Vote: I like it +62 Vote: I do not like it

As a tester, there are some beautiful ideas in the tasks, and I encourage everyone to participate!

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    3 weeks ago, # ^ |
      Vote: I like it +5 Vote: I do not like it

    Please share that glorious kompot with me!

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      3 weeks ago, # ^ |
        Vote: I like it +6 Vote: I do not like it

      I can, but I wouldn't recommend drinking it. Also, I can't, see link.

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        3 weeks ago, # ^ |
          Vote: I like it +5 Vote: I do not like it

        Opaa, not at all like kompot :/

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3 weeks ago, # |
  Vote: I like it +39 Vote: I do not like it

cf > ipl

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3 weeks ago, # |
  Vote: I like it +27 Vote: I do not like it

As a tester, I can assure you that this is definitely one of the rounds ever created.

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    3 weeks ago, # ^ |
      Vote: I like it +24 Vote: I do not like it

    as a participant, I look forward to it being one of the rounds of all time.

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3 weeks ago, # |
  Vote: I like it -21 Vote: I do not like it

as a participant, will I make it to expert?

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

Yaaay another round :D

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

    May God bless you with a positive delta in tomorrow's round!

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3 weeks ago, # |
  Vote: I like it +11 Vote: I do not like it

As a participant, I am 100% sure that the contest will happen. Hope it stays!!!

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

clashing with IPL final

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    3 weeks ago, # ^ |
      Vote: I like it +28 Vote: I do not like it

    No. IPL clashing with CF. Tell them to change the schedule xD

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3 weeks ago, # |
← Rev. 3   Vote: I like it +67 Vote: I do not like it

As a tester

Image
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3 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

As a participant hoping to get positive delta today :)

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3 weeks ago, # |
  Vote: I like it +7 Vote: I do not like it

Only the brave coders can participate in a contest on the night before an exam.

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    3 weeks ago, # ^ |
      Vote: I like it +14 Vote: I do not like it

    Only the brave coders can participate in a contest during an exam.

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

Need only +1

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

Hopefully, the problem statement will be short and precise just like the announcement!

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

wishing to get one step closer to cyan!

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3 weeks ago, # |
  Vote: I like it +7 Vote: I do not like it

Since the last 5 contest I'm stuck at 1100's hoping to cross 1200 this time. Best of luck everyone :)

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

The score of problem B is 1250!hoping to get positive delta :)

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

As a participant, I hope I solve 2nd problem quickly :(

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3 weeks ago, # |
  Vote: I like it +13 Vote: I do not like it
meme
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3 weeks ago, # |
  Vote: I like it -6 Vote: I do not like it

I sooo want to appear for this round, but same time IPL finals.

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

C looks buff

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

I think it's time to repeat all Stefan tricks... https://codeforces.com/blog/entry/117352

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

I rarely get any positive delta in Div 2. Hope this time is my time. :)

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

Hoping for a positive delta

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

cf>>>ipl

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3 weeks ago, # |
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Hoping to solve first two problems in short time and get some fun.

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

Hoping for Becoming cyan participant :)

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

1750 for C... interesting.

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

My confidence booster to achieve high positive delta

Alohomora
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3 weeks ago, # |
  Vote: I like it 0 Vote: I do not like it

500A to direct 1250B that is some score gaps i am seeing.

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

Scoring is wild on this one

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

Kontest > IPL hehe...

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

now, nikita is a boy!!!! its a girlllll

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3 weeks ago, # |
  Vote: I like it -58 Vote: I do not like it

Low quality problems especially C

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3 weeks ago, # |
  Vote: I like it -32 Vote: I do not like it

Ai can solve problem B like chatGPT 4. there are lots of people who use GPT4 to solve problem B. you can find many newbies who solved problem B but could not solve problem A. Xd I think the round should be unrated. and setters ensure that any problem can't be solved by any AI.

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    3 weeks ago, # ^ |
      Vote: I like it -24 Vote: I do not like it

    Exactly. Also a few youtube channels show the answers to the problems before the contest ends. It is unfair for those trying genuinely. I am not sure if there is any way to catch those who copied and those who tried on their own. Otherwise ratings of this contest will be false representation of ones capability.

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    3 weeks ago, # ^ |
      Vote: I like it -17 Vote: I do not like it

    I want the round to be unrated to dodge this -40 delta I'm about to get :(((

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

    It's already hard to check for AI solutions and also it's very hard to come up with div2A which will be unsolvable by LLMs.

    I think it's not feasible even now but in 6-12 months LLMs will be able to solve most existing div2A and div2B and some div2C so it will be almost completely impossible to prepare AI-resistant div2 contests.

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

      So are we near to the end of CP(atleast for those solving Div2 A,B & C)? Because most of the participants on codeforces are able to solve only Div2 ABC. If these problems are taken by AI then we will see only purple and above competing.

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        3 weeks ago, # ^ |
          Vote: I like it +6 Vote: I do not like it

        This is not the end of CP but will create some issues. Cheating is already a problem but it will be way more widespread soon.

        You can just ignore cheaters, e.g. compete with your friends or try to solve as much problems as possible. Or maybe at some day there will be alternative rating system which will be based on difficulty of tasks but not in comparison with other participants.

        Also, look at chess, they have much worse problem with cheating, it is not only more direct PvP game (so experience suffers from cheating more), chess AI on smartphones is way stronger than humans so the problem persists even at elite level.

        Nevertheless, chess is alive and well even though there is a sometimes heated discussion about cheaters and anti-cheating measures.

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

        AI compiles all the data from its dataset and gives you an answer. You may solve Div2A or Div2B (rarely) as it does not contain new things or logic. You can find that AI can solve many standard problems of various DSA topics, which are pretty tough and challenging to understand.

        So, for AI to solve problems, the person using AI must know how to solve the problem and what prompt is required to give to the AI model for it to create the solution. AI can help reduce your labour work but cannot think on your behalf. So, instead of crying about it, learn different topics and upskill yourself. Also, Idk if it's valid for you, but if you were unable to solve a problem, then it's your fault or lack of knowledge/skill. You also have the access to GPTs, you could have also solved the problem using the similar way.

        Just because you couldn't solve doesn't means that the round should be unrated

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

      Well may be ,if the language of problems is twisted

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    3 weeks ago, # ^ |
    ← Rev. 2   Vote: I like it +50 Vote: I do not like it

    I wasn't able to perform as well as others == The round needs to be unrated

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3 weeks ago, # |
  Vote: I like it +17 Vote: I do not like it

What a terrible problem distribution, I was performing at ~2000 rating despite having solved only 2 problems, and this was till last 30 mins of the contest.

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

Speedforces

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3 weeks ago, # |
  Vote: I like it +26 Vote: I do not like it

cant believe beat jiangly to problem E lmfao

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3 weeks ago, # |
← Rev. 4   Vote: I like it -23 Vote: I do not like it

.

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    3 weeks ago, # ^ |
      Vote: I like it +10 Vote: I do not like it

    $$$32$$$ is the number of bits, how you want to solve it?

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

    Because you need to be smart enough to not use brute force, which took me some time to realise

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3 weeks ago, # |
  Vote: I like it +26 Vote: I do not like it

I solved D using hashing. For each column there are $$$\leq n + 1$$$ ways to get only one $$$1$$$. And these "ways" are almost identical. (We should either remove all $$$1$$$ and recolor one $$$0$$$, or remove all $$$1$$$ except one)

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

is problem C using dp?

I tried tokenize each element by counting prime factor then greedy n^2 but didn't worked.

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

    Yes it's kind of dp, the main idea is that there are 2 options:

    1. LCM of whole subset is larger than max(a) -> then answer is n
    2. LCM of whole subset is <= max(a) <= 1e9 and then there are not that many possible LCMs, I don't have a proof but intuitively if you have a lot of different prime factors, then LCM quickly grows to be large and also that if you add a new number to a subset LCM either stays the same (does not increase amount of LCMs) or multiplies (so grows very fast).

    So the solution is: check for option 1, if it's not the case just keep dp = map<LCM, max_subset_size> and add elements one by one (iterate over all previously calculated LCMs) and update dp

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      3 weeks ago, # ^ |
      ← Rev. 2   Vote: I like it +18 Vote: I do not like it

      For the 2nd point, every possible LCM that can be formed from the subsequences will be a divisor of the maximum. So there are at most ~1000 divisors.

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

        Oh right, it's very simple, thanks!

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

      I used the same approach but got tle. Any particular reason you can think of?

      Edit: Got the correct answer was getting integer overflow. while checking intitial condition :(

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

        I had overflow as well initially, thankfully pretests cached it in my case

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

      Hey, I tried coding this out but got WA on test 8. Could you please tell me where I'm going wrong?

      Edit: I found my mistake. I did not account for the fact that sometimes the LCM could be so big that it overflows. I simply added a check to see if the LCM ever got bigger than $$$10^9$$$, and it ACed. Thanks!

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

can you give me hint for B please

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

E is very similar to this problem: https://oj.uz/problem/view/IOI23_longesttrip

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

    And yet knowing the solution of longest trip doesn't help you much

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

is C brute force try all subsets of factors of the largest element to see which subset will give the largest count

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

    Basically, yes. You first calculate the LCM of all the numbers. If that number is greater than $$$10^9$$$ then the LCM cannot be inside of the array, so you just take all the numbers. Otherwise, you iterate over every divisor of the LCM (works in $$$\sqrt{LCM}$$$ time), check for each number in the array if it's divisible, and then return the maximum result. Here is my submission: Submission

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

      good idea, thanks for sharing.

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      3 weeks ago, # ^ |
        Vote: I like it +7 Vote: I do not like it

      I think if the $$$LCM(array)$$$ $$$>=$$$ $$$max(array)$$$ is more appropriate than checking $$$>=$$$ $$$10^9$$$

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

      can you tell me what you are doing in this line? Thanks

      if (tl != x)continue;

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

        Let $$$x$$$ be the considered divisor of the LCM, and $$$S$$$ be the set of all numbers in the input such that $$$x$$$ is divisible by that number. The variable tl represents the LCM of the set $$$S$$$.

        For a subsequence to be valid, the LCM of that subsequence is not allowed to be in the input. Now, if the LCM and $$$x$$$ are not equal, there might be a chance that the LCM already is inside the input, so we just skip it. If the set $$$S$$$ was a valid subsequence however, then it would pass this constraint when the considered divisor $$$x$$$ is different, namely the same as the LCM of $$$S$$$.

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3 weeks ago, # |
  Vote: I like it -25 Vote: I do not like it

Yeah, this contest, its uh...., NOT GOOD.

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

Optimal approach for C?

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

    Find LCM of list. If LCM > max element: answer is n.

    Otherwise iterate through all factors of LCM.

    If factor not in list. Keep track of cnt of items divisible by factor and LCM of said items.

    Answer max(cnt of items for all factors not in list and where LCM(items) = factor).

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    3 weeks ago, # ^ |
      Vote: I like it +4 Vote: I do not like it
    • If LCM of array =/= largest value in array, answer is N
    • Iteratively calculate LCM of array. If at any point exceeds 10^9, we know the above case is automatically satisfied.

    Now, where LCM = largest value in array = L.

    The LCM of the elements for our answer can be only factors of L. So, find all factors of L.

    For each factor of L that does not appear in the original array, count how many maximum elements divide it. Ensure that LCM of these elements = this factor. If this is satisfied, find maximum across all count values.

    complexity: O(N sqrt(max(A_i)))

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

How to solve D, I was trying to solve it in O(n*m*min(n, m)), was getting TLE on pretest 19 :(

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    3 weeks ago, # ^ |
      Vote: I like it 0 Vote: I do not like it 
    i had a hashing soln but i couldn't submit due to small bug.
my idea is for each column compute all possible operation which can be made on i'th column  such that i'th column becomes special
there will be n such operation possible ops
pick the operation which occurs most of the times
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    3 weeks ago, # ^ |
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    Even I'm wondering how to D. I had some partial ideas , first of all take a particular column j , and make all it's elements zero , by choosing appropriate rows and inverting them. Then we can choose an i such that we will again apply inversion at the row i making this column good with 1 at row i. Now all the other columns , which had a 1 at i_th row and one more 1 at some other row will become good , but the main challenge seems to me is how to find them efficiently?

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

      let mask = j'th column which wan to make specail
      if you want i'th position to be 1 and all other positions 0, operation required is
      ops = mask ^ (1 << i)
      now you can use hashing to store all these ops and pick the most frequent ops

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    3 weeks ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    I saw your code and you have the right idea for small $$$m$$$.

    For small $$$n$$$ you can just iterate over all the $$$n$$$-sized bitsets that make each column have exactly one 1, group them in a map, and return the bitset that maxes the result.

    Code: 262803308

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

      Wow great!! Got it, that’s an amazing solution. I think if we use hashing along with your method, that would pass for any n and m. Thanks!

      Upd: I tried Hashing 262808152, getting wrong answer on test 4. Cannot really figure out why, I tried to debug it and there is a collision, 2 different bitsets are giving the same hash. I’ll try to figure out why. Also I tried submitting with multiple different values of mod and p but still the same result :(

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        3 weeks ago, # ^ |
        ← Rev. 2   Vote: I like it +13 Vote: I do not like it

        I was having the same issue, and I fixed it by having two keys with two different primes (I am using polynomial hashing).

        It is definitely not an elegant solution, and I see the other guys doing something with rand that probably provides a better solution (but I honestly do not understand it).

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          3 weeks ago, # ^ |
            Vote: I like it +8 Vote: I do not like it

          Damn, I never expected it was actually due to collision, because I tried so many values different values of mod. I thought there was some flaw with the algorithm to hash 2 vectors the same. I'll try double hashing then. Thanks!

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

            You can overcome this by again calculating the max value using the hash you found (changing the columns accordingly) . This gives correct answer (I implemented this) but cannot say why this works.

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

how to approach B?

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

    First convert x into binary form 10110100111.
    After that fix all positions where you have 11.

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      3 weeks ago, # ^ |
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      I'm not sure if it works, but I had two cases:

      1. We can see that if we have two consecutive digits that are 1's in binary we can increment i-1 and set i+1 to -1. For example if we have 3 in binary (11) this is also: 1*4 + -1*2 + 1*1.
      2. If i is -1 and i+1 is 1 then we can set i to 0 and i+1 to -1. For example, 3 = 1*4 + -1*2 + 1*1 = 1*4 + 0*2 + -1*1.
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      3 weeks ago, # ^ |
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      What about the number which has 29 1's like 2^30-1

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

    if x is odd we can turn x divisible by 4 by either adding 1 or subtracting 1, and if x is divisible by 4 the next element can be 0 meaning there can always be 0 between 2 non zero elements

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3 weeks ago, # |
← Rev. 4   Vote: I like it +1 Vote: I do not like it

i need 5 more minutes for D i am too noob at implementation. i had a small bug in code :(
upd : it's a correct AC soln

sad
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3 weeks ago, # |
  Vote: I like it +8 Vote: I do not like it

Even though I was only was able to solve 2 questions, I must say the questions were really interesting

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

Please someone explain Question : D Thanks in advance.

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3 weeks ago, # |
← Rev. 3   Vote: I like it +29 Vote: I do not like it

CHATGPT

ChatGPT give me correct solution to B

LOL

I think most of participants use ChatGPT to get the solution

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

    no, I didnt

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    3 weeks ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    many participants do cheat but the whole point of solving cp problems is to improve problem solving skills or to get dopamine boost after solving a problem which you don't get just by copy pasting code

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    3 weeks ago, # ^ |
      Vote: I like it -30 Vote: I do not like it

    Conest should be unrated

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

I love this round!

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

B is not easy :(((

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

    TBH this B = normal C, this C = normal D, this D = normal E. Tough round, but you need only 3 instead of 4 questions to get massive + delta.

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

    I just submitted it after the contest my bad :)

    My idea was something like this (I'm trying to prove it formally, but couldn't yet succeed) —

    If the binary representation has $$$x$$$ digits, then my claim is that — in the given constraints, we can build the array in almost $$$x+1$$$ digits. I wrote a recursive function to implement this.

    AC Submission: https://codeforces.com/contest/1977/submission/262782660

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3 weeks ago, # |
  Vote: I like it -49 Vote: I do not like it

Unbalanced contest, downvoted

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3 weeks ago, # |
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how to avoid tle in c?

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3 weeks ago, # |
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I thought the problems this contest were nice (and normal). For some reason the standings seem very unbalanced though, so probably my judgement is incorrect.

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

B was tough ,but as many participants said code of Chat GPT passes ,means many submitted it maybe you should check the question before, i lost my cool when i saw 9000+ submission, man i am bad at this but not that bad. Anyway if someone is interested in Non Chat GPTed solution then here it is 262781288

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3 weeks ago, # |
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during this round i suddenly realised that i have dyslexia

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3 weeks ago, # |
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Partial Editorial:

A — If n>=m and n and m share parity( both even or both odd) then yes, else no.

B: Repeatedly do the following until n = 0. If n is even. Output 0 and divide n by 2. If n is one more than a multiple of 4, Output 1 and divide n by 2. If n is one less than a multiple of 4, Output -1 and add one before dividing by 2.

C: If LCM(array)> max(array) answer is n.

Else, iterate through max(arr)'s factors,

For each factor track amt of items in list divisible by factor and LCM(those items).

Answer = max(amt of items if factor not in array and LCM(amt) = factor).

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    3 weeks ago, # ^ |
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    Is it enough to go through max(A) 's divisors only? I went through all divisors of all elements

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      3 weeks ago, # ^ |
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      yes, it is enough to go through the divisors of max(A), because this is LCM(A). All of item's divsors are simply a part of divisors of max(A).

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    3 weeks ago, # ^ |
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    How do you get the idea that if $$$n\mod2==1$$$, decide the next digit based on $$$n\mod4$$$ ??

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      3 weeks ago, # ^ |
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      Looking at sample cases.

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

        Damn! I was just trying and figuring out how to build the array for the whole time in the contest and did not care about samples! Anyway, my approach is seemingly different from the above one (more complex). My claim is that for a number $$$n$$$ with binary representation consisting of $$$k$$$ digits, the answer can be constructed in at most $$$k+1$$$ digits. I'm trying to prove it formally but couldn't do it so far. Submission: https://codeforces.com/contest/1977/submission/262782660

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          3 weeks ago, # ^ |
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          It must be at most n+1 digits. Proof by contradiction.

          If n+2th digit is 0. Then it serves no use.

          If n+2th digit is 1. Then number is at least 2^(n+1) + 1 which is impossible as number only uses n bits.

          If n+2 digit is -1. Then number is negative unless a higher digit is 1, which leads to same problem as described when n+2th digit is 1.

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            3 weeks ago, # ^ |
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            A non ChatGPT-ed simple solution for problem B:-

            If n%4==0, representation looks like "0 0 <that of n/4>"

            If n%4==1, representation looks like "1 0 <that of (n-1)/4>"

            If n%4==3, representation looks like "-1 0 1 <that of (n+1)/4>"

            If n%4==2, representation looks like "0 <that of n/2>"

            It meets all the conditions required.

            Here's my submission :- https://codeforces.com/contest/1977/submission/262746874

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    3 weeks ago, # ^ |
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    Looks like there are many ways to solve C. Mine is exactly the same as your editorial lol.

    262772972

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3 weeks ago, # |
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solved B literally 3 minutes after the contest ended :(

i should have typed a bit faster or something

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3 weeks ago, # |
  Vote: I like it -79 Vote: I do not like it

Hi Stefan2417, in the problem C I have made a small error in factorisation (instead of checking till $$$\sqrt{n}$$$, I only checked till $$$\lfloor\sqrt{n}\rfloor$$$), this caused my solution to be wrong for some cases, one of them is: n=4, a=[1,2,3,36]

If possible kindly rejudge my solution 262765545, I hope I will be cautious next time to not make these mistakes, at the same time I feel it would be unfair if I got a good rank due to this small error.

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    3 weeks ago, # ^ |
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    Why do you care about rating? =)

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    3 weeks ago, # ^ |
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    WA is WA.

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    3 weeks ago, # ^ |
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    Ahh... bad luck. I used to have this feeling 10 years ago (feeling that such a small error should be allowed to rejudge, or why do I have to suffer for such a negligible mistake where my logic/thought process is correct), so I totally understand your frustration. Thanks for making me nostalgic.

    Lesson: Error is error. 100% your fault. Own it, embrace it, learn from it, overcome it, grow from it.

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      3 weeks ago, # ^ |
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      Yes agreed, I owned up to my mistake in the comment, can anyone explain why am I getting downvotes? Have I offended someone?

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        3 weeks ago, # ^ |
          Vote: I like it +18 Vote: I do not like it

        people misunderstood you, thats all. Also dont worry about it, weak tests happen often. I would say you definitely deserved it as you got the problem idea right which imo is much more important.

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    3 weeks ago, # ^ |
    ← Rev. 2   Vote: I like it +5 Vote: I do not like it

    Ok, so what I think is that people probably misunderstood you: You were saying that your solution was incorrect because of a small typo, but it got marked as correct during the contest, because of which you got a good rank which you feel like you didn't deserve, correct (instead, they assumed that you were saying that your solution got WA because of a small bug, and so you should get AC just because it's so small)? Well, I'm saying that you do deserve it: Just look above / below you, there are so many people with solutions that aren't actually correct for D, but they work on all the testcases. In the end, in competitive programming, what matters is passing all the tests, not whether your solution is actually correct (although for most problems with strong tests, those two are equivalent...). So, if you deserve your rating, you will keep it in the next contest, and if you don't, well you're gonna drop down next contest anyway, so it's fine :)

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3 weeks ago, # |
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Damn, I couldn't even solve B when ChatGPT could :(

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3 weeks ago, # |
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MY submission 262742856 was accepted during the round but during testin g phase it got runt ime error on TC 1?I changed my language to c++ 20 and it got accepted.My question is why was it showing accpeted during the contest then? please look into this Stefan2417

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3 weeks ago, # |
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scoring worse than last div2

how is that even possible...

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3 weeks ago, # |
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I have seen many solutions that seems to me ai generated, Is there a way to report such solutions?

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3 weeks ago, # |
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solving two gray problem fast was sufficient to become an expert in this round

interesting

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    3 weeks ago, # ^ |
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    and getting three problem quickly is equivalent to master.

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

In problem D, is it true that atleast one of the first 39 columns should be special for the max case?

If not, hack : 262784406

UPD : Hacked :)

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3 weeks ago, # |
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short solution for D till editorial is not out
for each column consider all possible ops that can be performed to make i'th column special there will be n such ops possible for each column pick the operation which occurs most of the times ops can be store in the form of hashes

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3 weeks ago, # |
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Weak tests on Problem C, my solution is wrong shouldn't pass but it did.
Instead of checking if the whole LCM of the array is greater than the max. I just keep using modulo to avoid overflow, I used map to generate all possible LCMs hoping it wont TLE and I didn't.
It's easy to hack my solution just generate an array which it's LCM Mod 1e9+7 already exists in the array.

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    3 weeks ago, # ^ |
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    $$$a_i \le 10^9$$$

    so how can the LCM be greater than the MOD and in the array at the same time?

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    3 weeks ago, # ^ |
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    Why didn't you just put a condition to check if lcm gets larger than 1e9 and output n in that case?

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      3 weeks ago, # ^ |
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      Because I made my submissions in the last 5sc. I tried to fix it after a WA I knew the LCM was overflowing and didn't have time to think about the condition.

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

    no, your solution can't be hacked since it is correct) you are searching for all possible LCMs only if LCM of elements is already in array, so it is less then $$$10^9$$$, and obviously all possible LCMs of any subsequence are divisors of global LCM so there are less than $$$\sqrt{10^9}$$$ possible LCMs hence your time complexity is $$$O(n \sqrt{maxA})$$$ (oh I see you find global LCM wrong way and solution already got hacked :(( )

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      3 weeks ago, # ^ |
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      Yes I was talking about the way I found the global LCM, now I got hacked. anyway thanks for the analysis you made.

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    3 weeks ago, # ^ |
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    The tests are weak indeed. My solution clearly fails this test case

    1
9
2 3 6 7 1003 1003 14042 21063 42126

    Answer should be 5

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3 weeks ago, # |
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Please donate me +4 i want to be 1700 (crying emojis)

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    3 weeks ago, # ^ |
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    My prayers were not unheard , they increased 96, it was +75 before Happy happy to reach 1700

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3 weeks ago, # |
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In one sentence: the correct solution B is hidden in the tests.

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    3 weeks ago, # ^ |
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    Depends how are you approaching it to be honest, there are more ways than one to solve it.

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3 weeks ago, # |
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For problem B, my solution didn't pass the pretest. However, I think my output was correct if I'm not mistaken. I might be hella dumb but here is my answer for pretest 1 case 7:

input:
19
output:
6
-1 0 -1 -1 0 1

32 — 8 — 4 — 1 = 19
I don't know please help I'm a noobie.

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    3 weeks ago, # ^ |
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    You can't have two nonzero numbers beside each other.

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    3 weeks ago, # ^ |
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    There are 2 consecutive -1 in -1 0 -1 -1 0 1 so this is invalid.

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3 weeks ago, # |
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Can anyone explain to me why my submission 262785189 is accepted. I have read others and tried it out. It was correct. I do not understand why this is.

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3 weeks ago, # |
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The Test Cases are weak in D!

I designed a code to solve the problem which should solve the problem with O(m * m * n) time complexity! But the code passed with O(45 * m * n) time complexity, though it shouldn't pass!

Here is my solution: 262786847 Time: 249ms, Memory: 6976 KB

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3 weeks ago, # |
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Code Is this hackable?

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3 weeks ago, # |
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Can anyone tell me how I can improve? Seeing my recent performance in the past 2 contests, I seem to have hit a roadblock.

My C++ concepts are limited, and don't know much about DSA either. Should I stop participating in the contests for now and focus on learning or just continue practising from the problem set?

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    3 weeks ago, # ^ |
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    Continue practicing on the problemset. Solve problems around your level and you will be good.

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3 weeks ago, # |
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Guys please help. I AC'ed on B during the contest, but I got a TLE during system check. But when i submitted the exact same code but without fast io, it AC'ed.

Can anyone explain why it happened?

With fast io 262763870 Without fast io 262791262

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    3 weeks ago, # ^ |
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    cout << nums.size() << "\n";

    You can replace "\n" with '\n' and submit again.

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      3 weeks ago, # ^ |
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      How is that supposed to fix it? P.S. still TLE

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        3 weeks ago, # ^ |
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        According to this endl or "\n" will make the program flush, but even after replacing it still gives TLE then maybe because of another reason.

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    3 weeks ago, # ^ |
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    I think the actual problem in your code is RTE

    for (int j = 1; j <= len; j++){ nums[i+j] = 0; }

    In this line of code, i + j can be equal to nums.size(), which will eventually cause out of bound. Still I don't understand why removing fast I/O fixed, maybe due to the vector capacity allocation.

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3 weeks ago, # |
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editorial please. How to solve D?

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    3 weeks ago, # ^ |
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    Use hashing. For each column, only $$$n$$$ possible sets of rows can make it special, and all of them can be calculated quickly since they are very similar (any two sets only differ in 2 rows). Then just find the max frequency hash.

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

My solution to C by factorization and brute-force: 262772972

First, factorize each input numbers to product of some a_i ^ p_i, and maximize each p_i to get the LCM.

If LCM is greater than the largest input, the answer is trivial (n), otherwise:

Let's brute-force each p_i, i.e. brutu-force each (including non-prime) factor of LCM:

  • This factor should not occur in the inputs.
  • Choose all inputs that are divisible by this factor.
  • Verify that the LCM of the chosen inputs are this factor.
  • If all the above is satified, then we consider this as a proper answer. Maximize the count of chosen inputs during the brute-force.

Integers below 10^9 with the most count of factors is 735134400, which has 1344 factors. Time cost: 1344 * n * (count of prime factors) + cost of factorization

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    3 weeks ago, # ^ |
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    nice solution, some of my friends had a solution which got accepted in the contest, but later when I tried, failed on test case 18

    Your solution is absolutely correct

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      3 weeks ago, # ^ |
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      I just noticed that we only need to enumerate the factors of the largest number, all the remaining things can be easily solved by plain gcd() and lcm(). Factorization can be totally skipped.

      Anyway, the core logic is the same: enumerating factors and brute-force.

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3 weeks ago, # |
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I reached Pupil but suddenly I'm seeing that the rating decreased by 9 and I'm stuck in 1199 ....Why did it happen

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    3 weeks ago, # ^ |
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    Seems like they got to know that u r a master at using chatGPT :)

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3 weeks ago, # |
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Thanks for contest! C and D were not that great but B was nice and E is amazing!

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3 weeks ago, # |
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how do hacks work in div 2 contests?

a few people got hacked, but those hacks were added only after the contest ended and not while sy

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3 weeks ago, # |
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Round was very interesting, thanks!

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3 weeks ago, # |
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This submission is giving wrong answer for this test case:

1

10

2 3 4 4 4 4 6 12 100003 1200036

But it still got Accepted.

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    3 weeks ago, # ^ |
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    i checked it, yeah it gives wrong answer, the correct answer should be 6

    [2, 4, 4, 4, 4, 100003] -> 400012

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3 weeks ago, # |
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editorial when

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3 weeks ago, # |
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when do we get the editorial for this contest ?

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3 weeks ago, # |
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https://codeforces.com/contest/1977/submission/262831476

Why is this code giving WA in test4

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    3 weeks ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    Try using two hashes. One mod will give a high probability of failing if it's small, so hash as a pair of two integers each with a different mod. You can also use a larger mod if you want

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3 weeks ago, # |
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in my compiler, my code is 100 % right but when I submit it in the CF with the same compiler then it will show an error on a particular problem what do I have to do any idea???

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3 weeks ago, # |
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thanks

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3 weeks ago, # |
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Why no editorial yet ?

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3 weeks ago, # |
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يا زنديق انت واياه راوند 949 وقت الصلاة بلا شغل زندقة

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

In 1977C - Nikita and LCM few solutions have been accepted but on the testcase

n=8

arr=2,2,2,3,35,35,35,210

they are giving wrong answer maximum length subsequence is 6, they are giving 4, please add this testcase in main tests.262768289, 262777464

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3 weeks ago, # |
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Upsolved E. Beautiful problem indeed, had fun solving it.

Enjoyed all problems from B-E.

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

For question 5, Can someone please tell what will be the output of this graph:

Test Cases : 1.

Number of nodes : 5.

Edges: 4 which are 3 -> 1, 3 -> 2, 4 -> 3, 5 -> 3.

I tried dividing them in black and white groups but just can't, thank you for your help.

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

    Your graph does not match the requirements. I found that for $$$i,j,k = 1,2,4$$$ there is no edge at all for example.

    Edit : I was wrong about the statement. See me second answer.

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      3 weeks ago, # ^ |
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      Thanks for the reply but isn't 1 reachable from 4 ? Similarly, 2 is also reachable from 4. So I think this graph can be valid or maybe I am just dumb.

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        3 weeks ago, # ^ |
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        It seems I misunderstood the statement, thanks a lot ! For your question, an answer is then :

        1 3 4 of color 0

        5 2 of color 1

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          3 weeks ago, # ^ |
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          Yes, this coloring actually satisfies. I overlooked the fact that the black and white chains formed can intersect too.

          I have been thinking about the answer for hours now, thank you very much !!

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3 weeks ago, # |
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Attention!

Your solution 262608924 for the problem 1975A significantly coincides with solutions arka2005dey/262531402, _Sharma_Harsh/262608924. 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://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.

My solution for problem 1975A coincides with someone. I have got a false positive. Kindly look into it. @MikeMirzayanov

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3 weeks ago, # |
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So where is the tutorial?

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

delete me from this contest Stefan2417 alexchist i copied code from others , i accept it , i copied code from 262772037, delete me from this contest and dont delete the other person from contest

please look into this , i accept that i copied and the otherone didnt copy , MikeMirzayanov

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3 weeks ago, # |
  Vote: I like it -6 Vote: I do not like it

I was having with my personal compiler so during contest I used the online compiler ideone.com , after the contest I understood that my code got leaked

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3 weeks ago, # |
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When will the editorial be out ?

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3 weeks ago, # |
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I got a message saying it matched with some other people. My code is solely written by me and not shared with anyone else during the contest or before the contest. I wrote the code on my own and i am aware of each and every logic implemented. I truly dont know how it got plagged like this. I request you to look into it. If i were to explain the code, I can perfectly do that. Please help me regarding this. i will try not using any online compilers which might have caused to have the same syntax, but I didnt copy any code that is made available online. Please help

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

Very cool problem D!

I initially thought the solution will be something complex, but then I found the trick :)

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3 weeks ago, # |
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Spoiler
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    3 weeks ago, # ^ |
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    There are ways to find a mod hack or any constant base or mod, but I’m not sure about the details. Use random base and mod in Codeforces Contest.

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

C is harder than usual, anyways nice contest.

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3 weeks ago, # |
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is this contest unrated ?

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    3 weeks ago, # ^ |
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    I can't find my rating. I got it yesterday, but today it's zero. Is this correct?

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      3 weeks ago, # ^ |
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      No it's not ! You'll get back your ratings .

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

Problem B could be solved by GPT, and many of the user took the same approach as of me, but why is only my solution skipped. The solution to the problem is obvious and there is no other way of solving the problem that i could think of. Please do reconsider my solution , i think it should be rechecked kindly look @MikeMyrzayanov

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    3 weeks ago, # ^ |
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    same happened to me although my solution is by far not even close to other solutions

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3 weeks ago, # |
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Can anybody help me with my solution for problem C?Cannot identify where I went wrong.262825603

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3 weeks ago, # |
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My solution to problem B was skipped. I took some advice from GPT and it gave me an idea for the solution and I implemented it. I do not think that this is considered cheating because the solutions to the problem are similar and everyone may write them by coincidence. Is using GPT considered cheating?@MikeMyrzayanov

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3 weeks ago, # |
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my solution is not even close(mine's original) to the one that you are accusing me of and my ratings are gone now. Not only for this contest, but also for the last one. This is the second time this happened.

@MikeMirzayanov

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3 weeks ago, # |
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The test cases of problem C are weak and many solutions are hackable.

1
8
2 2 2 3 35 35 35 210

Many solutions give answer for above test case as 4 while it should actually be 6.

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    3 weeks ago, # ^ |
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    Oh god, you are right. I uphacked nearly 20% of the Python 3/PyPy 3 AC submissions made during contest with it. The problem is, rather than the fact that the test is weak itself, that most of those 'hackable' solutions were submitted in a long sequence in a short amount of time and their code looked almost the same; i.e. possibly a massive amount of cheats.

    (not saying all of them are cheats, but probably worth investigating in them)

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3 weeks ago, # |
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Can anyone give a detailed analysis of the hashing error rate of D ? I tried three types of hashing: (1) 64-bit xor hashing (2) 32-bit xor hashing (3) 64-bit sum hashing with automatic unsigned long long overflow. Only the (1) got accepted. Why?

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3 weeks ago, # |
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Why is my account rating not given. I have briefly explained you that i didnt copy any of the code. I worked hard solving those three questions. Why did they plag it and removed the rating. This is unfair. Please help me. I have solved those and submitted them on my own. I dont want this. Please help me.

@MikeMirzayanov

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3 weeks ago, # |
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My submissions was skipped. I have submitted with help of a book which was published before the contest. What can I do now??

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3 weeks ago, # |
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I solved 1977B - Binary Colouring problem of Div 2 category. I think my solution fulfils all the requirements of the problem, although it's been labelled as Wrong answer on test 1. The question says that in case of multiple solutions I can print any one of them. So can someone please help me on how to request for review of my submission. Submission Id: 263005014 Stefan2417.

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    3 weeks ago, # ^ |
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    For x = 11 your code outputs -1 0 1 1 in custom invocation, which is incorrect

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      3 weeks ago, # ^ |
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      (-1)*2^0 + 0*2^1 + 1*2^2 + 1*2^3 = 11

      Then why it's wrong ? The question says that if multiple solutions are possible, I can print any one of them.

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        3 weeks ago, # ^ |
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        You can't have two consecutive non-zeros in your output. Read the last condition and the announcement.

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3 weeks ago, # |
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Number b problem of this round , my submission get Skipped, while i do it in own. In other round, i see many times b, c problem the approach matched one to other but it's ok, but what happened this round ? So many skipped because of same approach? i think codeforces should check the account history before going to give skipped. I know scammer are rising day by day but it's really frustrating for programmer like us who struggle to get success. At last, really sad thing

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3 weeks ago, # |
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I became Pupil; But after the roll back, I am back to Specialist!!! Thank you.

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