vovuh's blog

Codeforces Round #786 (Div. 3)
By vovuh, history, 3 years ago, In English

Hello! Codeforces Round 786 (Div. 3) will start at May/02/2022 17:35 (Moscow time). You will be offered 6 or 7 problems (or 8) with expected difficulties to compose an interesting competition for participants with ratings up to 1600. However, all of you who wish to take part and have rating 1600 or higher, can register for the round unofficially.

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

You will be given 6 or 7 (or 8) problems and 2 hours to solve them.

Note that the penalty for the wrong submission in this round (and the following Div. 3 rounds) is 10 minutes.

Remember that only the trusted participants of the third division will be included in the official standings table. As it is written by link, this is a compulsory measure for combating unsporting behavior. To qualify as a trusted participants of the third division, you must:

  • take part in at least two rated rounds (and solve at least one problem in each of them),
  • do not have a point of 1900 or higher in the rating.

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

The problems were invented and prepared by Adilbek adedalic Dalabaev, Alexander fcspartakm Frolov, Ivan BledDest Androsov and Mikhail awoo Piklyaev. Also huge thanks to Mike MikeMirzayanov Mirzayanov for great systems Polygon and Codeforces.

Also huge thanks to ashmelev, Vladosiya, mesanu and I.AM.THE.WILL for testing the round and valuable feedback on the problems!

Good luck!

UPD: Editorial is published!

  • Vote: I like it
  • +141
  • Vote: I do not like it

Comments Comments (173)
| Write comment?
»
3 years ago, # |
  Vote: I like it +21 Vote: I do not like it

Good luck everyone!

→ Reply
»
3 years ago, # |
  Vote: I like it +40 Vote: I do not like it

omg vovuh div3s are back!!! :D

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Yes, I hope no smurfs and good luck to everyone!

→ Reply
»
3 years ago, # |
  Vote: I like it +23 Vote: I do not like it

Best way to celebrate my birthday!! Vovuh's Div3s are lit!

→ Reply
»
3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

I have been participating in Div3 for quite a long but this contest never went unrated for me. I wish it doesn't happen again after this contest.BTW, Best of luck everyone.

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +41 Vote: I do not like it

Best Wishes everyone for the contest and Happy Eid to all of you.

→ Reply
»
3 years ago, # |
  Vote: I like it +27 Vote: I do not like it

vovuh, welcome back

→ Reply
»
3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

I have a question why some of the announcements don't give us exact numbers of problems?

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I think because they don't have a final problemset yet, maybe because they have 8 or more problems made but they are not sure which problem to put on 1-2 places, and they will decide later

    → Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

hopefully i get specialist yoooo

→ Reply
»
3 years ago, # |
  Vote: I like it +5 Vote: I do not like it

Eid Mubarak today

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +1 Vote: I do not like it

Did someone notice, its Codeforces Round #786(the holy number) on the eve of Eid-al-Fitr!!! Such coincidence Much wow!

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

vovuh welcome back in div 3s

→ Reply
»
3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

welcome back in div 3s, vovuh

→ Reply
»
3 years ago, # |
  Vote: I like it +6 Vote: I do not like it

You will be offered 6 or 7 problems (or 8)

or 9

→ Reply
»
3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Eyeing specialist from a distance 0_0

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Hope I will be specialist after this div3 contest!!!

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I want be specialist after this contest.

→ Reply
»
3 years ago, # |
  Vote: I like it -8 Vote: I do not like it

vovuh welcome back

please make the time of the contest 2 : 15

→ Reply
»
3 years ago, # |
  Vote: I like it +2 Vote: I do not like it

Long time these authors have not done div 3. All good solutions!

→ Reply
»
3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

I think this will be a div 3 round.

→ Reply
»
3 years ago, # |
  Vote: I like it -38 Vote: I do not like it

Very Bad contest

→ Reply
»
3 years ago, # |
  Vote: I like it +42 Vote: I do not like it

Marinush

→ Reply
»
3 years ago, # |
  Vote: I like it +21 Vote: I do not like it

Why there is not any example or description note for test cases in problem F... can't really understand the test cases :(

→ Reply
»
3 years ago, # |
  Vote: I like it +19 Vote: I do not like it

Spent 30 minutes to understand F :(

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

I has closed div 3. Very good problems! Upd: hacked E :(

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I think D is the hardest problem :))

→ Reply
  • »
    »
    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    what was your approach? I tried like this but failed — the pairwise mins in array A must have an increasing order. for odd n the pairing excludes the first element, but it has a constraint to be the smallest integer.

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it +1 Vote: I do not like it

      sort each pair of (N, N — 1), (N — 2, N — 3).... and check if you can get sorted array. your solution does not work for 2 4 3 5.

      → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Orz

    → Reply
»
3 years ago, # |
  Vote: I like it +20 Vote: I do not like it

G is a simple but educational problem. Hope there will be more problems like this in the following div2/3 contests.

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Can you explain it please?I think it's the longest path in the graph after deleting some edges but there are some cases that I choose edges that I shouldn't

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
      ← Rev. 2   Vote: I like it 0 Vote: I do not like it

      It's a simple dp problem where you first topsort the vertices, and then for each vertex $$$u$$$ with in-degree greater than 1, you iterate over the parent nodes $$$v$$$ ($$$v \rightarrow u$$$), and if the parent node has out-degree greater than 1, you update the dp value as $$$dp_u = dp_v + 1$$$.

      → Reply
      • »
        »
        »
        »
        3 years ago, # ^ |
          Vote: I like it +6 Vote: I do not like it

        Since the graph is a DAG, you can use memoized search instead of topsort

        → Reply
»
3 years ago, # |
  Vote: I like it +2 Vote: I do not like it

E — Breaking the Wall, Wrong answer on test 32. What was that?

→ Reply
»
3 years ago, # |
  Vote: I like it +35 Vote: I do not like it

can u unfreeze the standings + let us submit

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Interesting tasks and good round! Thanks :)

→ Reply
»
3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

I did'n noticed that problem G statement:'a valid directed acyclic graph', I tried tarjan to shrink strongly connected components into points (poor english).Although it was unnecessary, I'm just wondering if this algorithm would be correct without such a condition.I would be grateful if someone could help me.

→ Reply
»
3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

Why results are frozen?

I can't submit((

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it
Spoiler

How it should work? It just reloads the page

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I spent 40 minutes on D and got TLE because I'm using native linked lists, there is no way you wanted me to implement an optimized liked list for this right?

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Nope, note that you can only change the relative order between two elements after sorting ($$$a_{n} \leftrightarrow a_{n - 1}, a_{n - 2} \leftrightarrow a_{n - 3}, ...$$$)

    → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +4 Vote: I do not like it

    you don't really need to do the operation.

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      can you explain how to calculate it then please

      → Reply
      • »
        »
        »
        »
        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        do some simulation and you will find you only can pairwisely revert elements from the right side.

        → Reply
      • »
        »
        »
        »
        3 years ago, # ^ |
        ← Rev. 5   Vote: I like it 0 Vote: I do not like it
        Problem D
        → Reply
»
3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

vovuh rocks!!

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

anyone explain F pls

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    remember the total number N of current icons on the screen, and the number of icons in the first N cells. The result is the difference between them. Use fenwick tree to find out the second value.

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Actually there is no need for a Fenwick tree. You can simply iterate through the columns for each query and sum the amount of operations needed

      → Reply
      • »
        »
        »
        »
        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Looks like with this time complexity it will work only for c++

        → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    You can consider unraveling the matrix resulting in a vector (i.e. taking the first column, then second, then third, and stick them to make a new vector). Then, the apps need to be in a prefix of this vector. when adding a image, you extend the prefix, otherwise you need to shrink the prefix (where you need to move the icons). Then, to find how many images you need to move is to calculate the number of images you have in the prefix you currently are in. If this number is X, and you have Y images on the desktop, then $$$Y - X$$$ is the answer. To calculate X, you can do some if's when erasing/inserting images (to see if they spawn/despawn directly from your prefix)

    → Reply
  • »
    »
    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it 0 Vote: I do not like it

    You had to count gaps in the icon arrangement using segment/fenwick trees.

    Video Solutions: A-F

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
      ← Rev. 2   Vote: I like it 0 Vote: I do not like it

      No need of segment tree or fenwick tree it can be done without any extra log factor just have a two variables currow curcol which will denote the position for last icon after rearrangement. And whenever a new * is added somewhere just change the curpos to next postion by curcol+=(currow+1)/n currow=(currow+1)%n; and calculate total stars and number of stars inside the consecutive region. Submission- https://codeforces.net/contest/1674/submission/155735790

      → Reply
»
3 years ago, # |
  Vote: I like it +78 Vote: I do not like it

Aight, guys. This comment may seem a bit toxic or something, but I still want to express how do I feel about this situation. Firstly, I want to say that the problem F is mine (I'm an author of the idea and I prepared this problem).

I get when people are confused about palindrome bracket sequence (as it was in some recent Educational Round), this thing can be really confusing when you see it. But I can't get how people are not familiar with the order of icons appearance on the desktop. Like, if you code from your phone or from fedora or some other OS without GUI, the desktop itself for you can be really confusing, but I don't think there are a lot of such people among Div.3 participants. So, I can't believe nobody saw that when you install some application and mark the field ''add the icon to the desktop'', this icon appears in the first column that is not full, and then in the first position from the top inside this column. Also, when you sort your icons, they're rearranging exactly as in this problem. I thought I created a very life-based problem and people will understand it easily, but I was wrong.

Moreover, the statement wasn't perfect, but was clear enough. There was the following sentence in the second paragraph: ''In other words, some amount of first columns will be filled with icons and, possibly, some amount of first cells of the next (after the last full column) column will be also filled with icons''. I agree that I could add some pictures or maybe highlight some words in the second paragraph, but I thought this is very-very life-based thing that anyone will understand without problems.

At last, I agree that I'm not completely right in this situation, because our task is to make statements as clear as possible for anyone. But, as I said before, this is just my personal opinion, so you can discuss it with me here or maybe just downvote this comment if you disagree.

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +21 Vote: I do not like it

    The statement is a bit hard to understand, but anyway it's still OK to me.

    → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +29 Vote: I do not like it

    Hey, thanks for the problem, and for the years of service!

    I think the statement was okay, but when reading it for the first time, I felt like an image with the statement would be perfect. Or maybe, explanation of one test case.

    → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +12 Vote: I do not like it

    As a tester, I agree with vovuh. I also thought everyone encountered desktop arranging at least once and would understand the problem perfectly. To me, it seems like a very nice life-based problem so I didn't suggest any changes to the statement. Will definitely keep such things in mind from now on though.

    → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    For problem F, why did the question allow a time limit of 3 seconds? It'd be reasonable to make the participants to use a time limit of 1 second instead.

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it +4 Vote: I do not like it

      Yeah, that's probably true. I was just scared that some python/java/(any other non-CP language) users couldn't solve this problem because there is too many IO. But I was completely wrong, so there are also some $$$O((n+m)q)$$$ solutions in C++, and this is a bit disappointing. I also discussed this with Ivan, and we decided that 3 seconds is okay, but turned out it wasn't.

      → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    "But I can't get how people are not familiar with the order of icons appearance on the desktop."

    On mac, by default it is right to left:

    → Reply
»
3 years ago, # |
  Vote: I like it +28 Vote: I do not like it

Problem E's pretests are so weak!!!!!!! I have already hacked myself!!!!

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Educational rounds and div3s are meant to have weak pretest. Why do you think there is a 12 hour hacking period?

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it +15 Vote: I do not like it

      But it is the responsibility of questions setters to have strong pretests that contains most situation of the problem,rather than called the competitors to successfully hack more than one thousand accepted answer.

      → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +6 Vote: I do not like it

    Dear contestant...

    Please don't worry if you had to hack yourself in order to see if your solution is good. In a programming competition, the tests used can be useless, you should have assured your code works. Sorry if this does not fit in your view.

    Author of xem sdod

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it +30 Vote: I do not like it

      Imposter?

      → Reply
      • »
        »
        »
        »
        3 years ago, # ^ |
        ← Rev. 2   Vote: I like it +5 Vote: I do not like it

        Dear non-contestant,

        Please don't worry if someone else is a fan of EGOI and swapped two letters from your name. In a programming community thoughts as such are completly irrelevant. Apologies if this does not fit your view.

        So, can we please get back to programming ?

        → Reply
»
3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

Thank you vovuh and the team for organizing this Div 3 contest. I love the questions and how they were being framed.

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +6 Vote: I do not like it

Amazed to see some red+ coders getting hacked on E. What is the hack case ?

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

thank you AHMADUL for solving A!!!

→ Reply
»
3 years ago, # |
  Vote: I like it +12 Vote: I do not like it

in D I thought that the operation from b to c was the same as from a to b. F :(

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    I spent about 30 minutes thinking of a solution for this problem before I realize It was not the real problem

    → Reply
»
3 years ago, # |
← Rev. 5   Vote: I like it +22 Vote: I do not like it

Testing data for problem E is far too weak.

We can hack a lot of solutions using this data.

2
1 5
→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

idea for d? and whats on pretest 32 in e?

→ Reply
»
3 years ago, # |
  Vote: I like it +14 Vote: I do not like it

so many hacks to E...I have rised from 600+ to 400+.

→ Reply
»
3 years ago, # |
  Vote: I like it +8 Vote: I do not like it

;-; Got WA on C just because I wrote (1ll<<a.size()) as pow(2,a.length())

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Can someone explain to me, what's the point of the removing condition in G? Doesn't it allow us to remove any edge?

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +3 Vote: I do not like it

    The condition is that, after removing any number of edges, the final graph MUST have in-degree and out-degree of EVERY vertex to be strictly less than the original graph (except if the in/out degree was 0 for any vertex in original graph).

    For example, if there is a graph 1->2->3->4, then we can't just remove 1->2, because in the resulting graph (2->3->4), the in-degree and out-degree of 3 are the same as that in the original graph.

    → Reply
»
3 years ago, # |
  Vote: I like it +12 Vote: I do not like it

thanx for good and strong E test's!

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

any hint for E?

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +49 Vote: I do not like it

    I'm trying to explain my solution while my solution is hacked...so I deleted all my notes...

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it +23 Vote: I do not like it

      One minute I was hacking my junior, the next I was being hacked by myself...

      → Reply
»
3 years ago, # |
  Vote: I like it +31 Vote: I do not like it

I didn't noticed acyclic graph in G...

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +10 Vote: I do not like it

Wow tons of hacks on E. Accept number decreases in every second!

I forgot a corner case (1 10000 3), the answer is 2(shoot middle, then right). Weak sample and tests.

Upd: till now, about a half solutions hacked!

→ Reply
»
3 years ago, # |
  Vote: I like it +9 Vote: I do not like it

E's pretests are a joke

→ Reply
»
3 years ago, # |
  Vote: I like it +18 Vote: I do not like it

These hacks are crazy, I have gone from 170th to 89th in 45 minutes after the contest. Maybe I will be next...

→ Reply
»
3 years ago, # |
  Vote: I like it +134 Vote: I do not like it

Cursed spoiler:

open this spoiler
→ Reply
»
3 years ago, # |
  Vote: I like it +11 Vote: I do not like it
Only for Newbies
→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

What does E's pretest mean...I wrote this chmin(ans, (mx + mx + 1) / 2); Then it's got AC????? Of course it's hacked...

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +19 Vote: I do not like it

I am a simple man, I solved 5, my rank was initially around 1800, then everyone started getting hacked and my rank became around 1200 :) , and then ( you guessed it right ) I myself got hacked! now my rank is 2200+

Please CF don't troll me XD

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Problem E: My solution

Whats wrong with this?

→ Reply
»
3 years ago, # |
  Vote: I like it +41 Vote: I do not like it

2000 participants: "I solved E!" Joury: "No You didn't)"

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +8 Vote: I do not like it

    There are more than 50 pages of successful hacking to E. Never imagined to this...

    → Reply
»
3 years ago, # |
  Vote: I like it +12 Vote: I do not like it

The pretests are weak — I solved E, then got hacked. I forgot about the case a[i] > a[i + 1] * 2(and vice versa) (then it's not (a[i] + a[i + 1] + 2) / 3 but (a[i] + 1) / 2)). Such a simple one, but pretests didn't cover that case..

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    There are no pretests in div.3, div.4 or educational rounds. It's full feedback meaning submissions are judged on all tests. Although, the tests on E were weak no doubt. The solves went from ~3000 to ~550 after system testing was over.

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Thank you, didn`t know that

      → Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

interesting in hacking problem E the real game

→ Reply
»
3 years ago, # |
  Vote: I like it +10 Vote: I do not like it

Enjoyed the few minutes when I thought I solved E

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I have been hacked :(

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +7 Vote: I do not like it

Too weak E pretests!!

→ Reply
»
3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

proof by AC isn't working with problem E

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +1 Vote: I do not like it

This round is amazing ,Because some of my friends get higher ranking than one hour ago。

→ Reply
»
3 years ago, # |
  Vote: I like it +8 Vote: I do not like it
Meme
→ Reply
»
3 years ago, # |
  Vote: I like it +28 Vote: I do not like it

If everyone is hacked, it means everyone is not hacked.

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

I might be making a rude comment but the number of hacks makes me wonder that is the Jury's solution completely correct? Is it not possible for some really correct solutions to get hacked just because the jury's solution happens to be not perfect.

Just a thought.

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +13 Vote: I do not like it

    My dear friend, you are absolutely wrong jury always have a proof to defend their solutions

    → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    The hacks are tested against jury's solution, that means the jury solution gives different output from the hacked solutions. Which means it's correct.

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      Moreover, there is not just ONE jury solution, usually there are at least 5 to 10 different valid solutions (written in different languages)

      → Reply
»
3 years ago, # |
  Vote: I like it +4 Vote: I do not like it

"You should always prove your solutions before submitting" — Tourist

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it 0 Vote: I do not like it

Alright, for those that had wrong answer on pretest 32 in problem E like me, maybe this one will help you:
2
3 6
Answer is 3. I had 4 because I was trying to reduce by 3 both numbers while possible (applying two operations, subtracting 1 2 and 2 1) but this is not optimal, as you can just shoot number 6 three times.

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +40 Vote: I do not like it

Is it May's fool? :')

→ Reply
»
3 years ago, # |
  Vote: I like it +2 Vote: I do not like it

Problem E: failing just like the question name : Breaking the Wall (of pretests)

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +4 Vote: I do not like it

If you prefer command line, checkout CF Doctor for more flexibility and faster feedback.

If you are/were getting a WA/RE verdict on problems from this contest, you can get the smallest possible counter example for your submission on cfstress.com. To do that, click on the relevant problem's link below, add your submission ID, and edit the table (or edit compressed parameters) to increase/decrease the constraints.

If you are not able to find a counter example even after changing the parameters, reply to this thread (only till the next 7 days), with links to your submission and ticket(s).

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

A B C D E-Hacked

→ Reply
»
3 years ago, # |
← Rev. 3   Vote: I like it +6 Vote: I do not like it

I have just refreshed the page and 620 accepted solutions for E became 570(during 2min). I am sure till tomorrow there will not be correct solutions

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

What does "the prefix of the next column" in F means? Does it mean that I need to put all remaining icons to the upper side?

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Yes

    → Reply
  • »
    »
    3 years ago, # ^ |
    ← Rev. 2   Vote: I like it +10 Vote: I do not like it

    Yes. Effectively it means that if you took all the icons out, you would fill the columns greedily left to right, and within each column greedily top to bottom.

    Correct configuration:

    ***..
***..
**...
**...

    Incorrect configuration:

    ***..
**...
***..
**...
    → Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Can anyone help me know why i am not included in common standing. I do not have a rating of 1900 ever. Am I not trusted participant ?

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

do successful hacks give extra points for this round?

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    nope!

    → Reply
    • »
      »
      »
      3 years ago, # ^ |
        Vote: I like it 0 Vote: I do not like it

      really? Damn! I hacked 10 submissions. I guess it was a waste of my time... So why do problem setters provide 12 hours of hacking phase for participants? Do they want us to make strong pretests for them?

      → Reply
      • »
        »
        »
        »
        3 years ago, # ^ |
          Vote: I like it +19 Vote: I do not like it

        It was a waste of your time if you didn't learn anything from it. Otherwise it wasn't.

        Div 3 (and 4) rounds are run according to Educational Round rules. The purpose of the 12 hour hacking phase is to encourage the studying of others' code and learning how to stress test, debug, and spot holes in algorithms. Often the tests deliberately leave out certain cases to make hacks more likely (though rarely as extreme as problem E today).

        If you only did it for points then yes I suppose you did waste your time; otherwise you did exactly what the round is designed for.

        → Reply
        • »
          »
          »
          »
          »
          3 years ago, # ^ |
            Vote: I like it 0 Vote: I do not like it

          I meant to say that doing 10 hacks was a waste of my time since nearly all of them failed on same test case, and obviously I learned why those submissions failed

          → Reply
      • »
        »
        »
        »
        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        it's not a waste of time if you hacked people with better ranking than yours

        → Reply
      • »
        »
        »
        »
        3 years ago, # ^ |
          Vote: I like it 0 Vote: I do not like it

        Anyways, You don't get points for hacking in Div 3

        → Reply
»
3 years ago, # |
  Vote: I like it +3 Vote: I do not like it

hacks ;-;

→ Reply
»
3 years ago, # |
  Vote: I like it +21 Vote: I do not like it

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Was hacked in E because of the case for (i,i+1) we have equations

2*k1+k2>=a[i] 2*k2+k1>=a[i+1]

when k1<0 or k2<0 my solution was not working. This case should be in pretests!

→ Reply
»
3 years ago, # |
← Rev. 2   Vote: I like it +1 Vote: I do not like it

E: WA on pretest 14 and 32 -> could not even enjoy the temporary happiness

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

and if you had no participation and zero rating, then the rating will not rise?

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Could someone please try and hack 155731727

I want to see if I have written the correct solution. Thanks,

→ Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    all system tests (with hacks) are available now. if you pass it then solution is correct

    → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it 0 Vote: I do not like it

    Your solution is completely correct.

    → Reply
  • »
    »
    3 years ago, # ^ |
      Vote: I like it +1 Vote: I do not like it

    Thanks for the feedback

    → Reply
»
3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

I have a question when this contest is rated.

→ Reply
»
3 years ago, # |
  Vote: I like it +7 Vote: I do not like it

Here are video Solutions to all problems in case people are interested.

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Why I haven't get any rating after participating in the contest and solving 3 questions there?

→ Reply
»
3 years ago, # |
  Vote: I like it 0 Vote: I do not like it

Can you explain me why I haven't got any rating after participating in the contest and solving 3 problems there?

→ Reply
»
3 years ago, # |
  Vote: I like it +1 Vote: I do not like it

This F problem is not as difficult as F problem. Original two-dimensional problem can be very very easy converted to one-dimensional.

→ Reply
»
3 years ago, # |
← Rev. 4   Vote: I like it +1 Vote: I do not like it

Hello MikeMirzayanov,BledDest,vovuh,adedalic.System

I find out some plagiarism activity in Codeforces Round 786 (Div. 3).

Please check out submissions of problem 1674B - Dictionary,submissions are 155621247 and 155678427 They both have same function with same variable but just comment down it and wrote it again with some variable change. Please look out at it.

Proofs given below and also you can check submissions...

→ Reply