thanhchauns2's blog

Codeforces Round #812 (Div.2)
By thanhchauns2, 4 years ago, In English

Hi Codeforces!

GlowCheese, DeMen100ns, SPyofgame and I are delighted to invite you to participate in Codeforces Round #812 (Div. 2).

  • Start time: Aug/06/2022 17:35 (Moscow time)
  • Duration: 120 minutes.
  • Number of tasks: 6, including at least one interactive problem. Make sure to read this blog and familiarize yourself with these types of problem before the round!
This contest is brought to you by:

Special thanks to:

The score distribution is 500-1000-1750-2000-2500-3000

Hope to see you in final standings!

UPD: We have a small gift for a Vietnamese participant who have the highest score, so if it is you, please DM me after contest. Good luck everybody!

UPD2: Editorial

UPD3: Congratulations to the winners!

Div.2:

  1. RGB_ICPC7

  2. Xylenox

  3. 5cd

  4. Jason2022

  5. Imot

Div.1 + 2:

  1. peti1234

  2. A_G

  3. kotatsugame

  4. jiangly

  5. Rubikun

  • Vote: I like it
  • +845
  • Vote: I do not like it

Comments Comments (248)
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4 years ago, hide # |
 
Vote: I like it +91 Vote: I do not like it

As an author, love you SPyofgame

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4 years ago, hide # |
 
Vote: I like it +30 Vote: I do not like it

As a fan of idol thanhchauns2, I'm really looking forward to participating and solving his own contest. Hope everyone have a great time!

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4 years ago, hide # |
 
Vote: I like it +169 Vote: I do not like it

problems are great!

Spoiler
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4 years ago, hide # |
 
Vote: I like it +8 Vote: I do not like it

ᓚᘏᗢ

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4 years ago, hide # |
 
Vote: I like it +11 Vote: I do not like it

As a tester, I wish u guys could gain the rating :3

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

Finally your contest after 1 year,

Excited to see and all the everyone!!

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4 years ago, hide # |
 
Vote: I like it +19 Vote: I do not like it

As a tester, hope you guys to enjoy this round.

As a weeb, I recommend you guys Youzitsu (Light Novel) and Gabriel Dropout (Anime/Manga)

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

As a tester, I hate love DeMen100ns

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4 years ago, hide # |
 
Vote: I like it +8 Vote: I do not like it

hydroshiba orz

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4 years ago, hide # |
← Rev. 2  
Vote: I like it +8 Vote: I do not like it

As the biggest fan of DeMen100ns and SPyofgame, hydroshiba orz

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4 years ago, hide # |
 
Vote: I like it +30 Vote: I do not like it
As a tester...
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4 years ago, hide # |
 
Vote: I like it +6 Vote: I do not like it

Intentionally skipped testing this round, hope this one will be as good as previous one

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4 years ago, hide # |
 
Vote: I like it +10 Vote: I do not like it

The white text is painfully obvious on mobile

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4 years ago, hide # |
 
Vote: I like it +22 Vote: I do not like it

My best effort in this project is breathing instead of making instant-rejected problems (")>

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

Finally your round, obviously I will take part in it

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4 years ago, hide # |
 
Vote: I like it +9 Vote: I do not like it
  • "Last but not least, you for your participation and being WA, then dropping at least one color :P" It's the best easter egg I've ever seen :P
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4 years ago, hide # |
 
Vote: I like it +6 Vote: I do not like it

amogus

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

orz

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

why this announcement is published 6 weeks ago?

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

    Probably because that's when it was announced? It was only added to the Home page now because it's the next contest.

    There are three other contests scheduled that should also be added to the Home page when they're up next.

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

nice :>

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

As a tester, i worked with authors who are talented high school, college students, they had to make a lot of efforts to prepare problemset during months, some problems were rejected or removed to have the best problemset for contestants. So no matter what you feel about this round, plz upvote this contest to encourage young Vietnamese CP-ers to contribute global CP community. Finally, let's enjoy our problems and hope all you guy get good ranking.

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    4 years ago, hide # ^ |
     
    Vote: I like it +4 Vote: I do not like it

    yeah, you are right. Everyone who has made efforts to this contest deserve encouragement!

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

By the way, the kitty on the promotional picture is really cute (>w<)

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

Picture looks really good,hope it will be a nice round :)

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

I hope that C, D will not be as difficult as in the previous contest

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

Hope I can reach Master after this round! Good luck to everyone!

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4 years ago, hide # |
 
Vote: I like it +11 Vote: I do not like it

Seems like nobody notice that our coauthor, DeMen100ns has two colors in the announcement.

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

Hope everyone have fun!

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

Most colourful blog I've seen in a while :3

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4 years ago, hide # |
 
Vote: I like it +4 Vote: I do not like it

image-2022-07-06-T09-36-54-205-Z

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4 years ago, hide # |
 
Vote: I like it -66 Vote: I do not like it

Watch as people upvote me just because I am a GM

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4 years ago, hide # |
 
Vote: I like it +11 Vote: I do not like it

04bf86d84b4883c8e7e6e94ed23606ff

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

Wish I can solve at least one problem!

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

Good luck! love you SPyofgame

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

Again a contest with a huge gap between B and C :(

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

i would personally prefer two minutes later, but ok

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4 years ago, hide # |
← Rev. 3  
Vote: I like it -26 Vote: I do not like it

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

Is this rated for me? Up to 2100 or up to 1900

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4 years ago, hide # |
 
Vote: I like it +4 Vote: I do not like it

C 1750 looks scary :(

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4 years ago, hide # |
 
Vote: I like it +39 Vote: I do not like it

I will try to become Expert in this contest!

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

According to the points distribution, I guess C is going to be a good question. So what do u think ?

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

I will become specialist this contest!!!

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

Good luck for everyone ❤(✿◕‿◕✿)❤

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

As a participant, I wish myself good luck in advance

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4 years ago, hide # |
 
Vote: I like it +72 Vote: I do not like it

One of the authors is in a really bad health situation right now. To all participants, please shows your respect to him by solving as many problems as you can, we all want him to overcome as soon as possible.

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

| Last but not least, you for your participation and being WA, then dropping at least one color :P

B-but we can't drop any more colors

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

I guess the interactive problem is problem C because it has 1750 points :) Is my thinking correct or not ?

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

    As far as I know, ABC are never interactive since they only should require basic data structures and math knowledge sometimes (and their statement should be clear to everybody), the interactive problem will probably be D or F (my opinion :)), it's just because authors probably came up with this C, but they think it's a bit more difficult than usual, so they made it 1750

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

All the Best @EveryOne !!

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4 years ago, hide # |
← Rev. 2  
Vote: I like it 0 Vote: I do not like it

Hope I become pupil in this contest

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

leetcode overlapping contest :( Anyways will solve at least 1st on leetcode, just to get coins for a leetcode shirt :)

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

Stuck on problem A :holyfuck:.

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4 years ago, hide # |
← Rev. 2  
Vote: I like it 0 Vote: I do not like it

b and c are very good problems!

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4 years ago, hide # |
 
Vote: I like it +31 Vote: I do not like it

How does a blue coder solve f in 5 minutes? Jiangly didn't do it for 40 minutes.

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

How to solve E?

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

Video Solution for Problem C.

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

Hi,

I solved problem C at 1:14:31. But later i saw the code and felt that it might fail system testing as i used int instead of long at one place so i submitted again at 1:47:33 and as of now both are showing pretest passed. But i am getting points according to the 2nd attempt. If after system testing my 1st accepted solution does not fail so will i get score according to my 1st submission or 2nd submission?

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

    During system testing, your first submission will be skipped and your second submission will be considered. Also you'll lose extra 50 points for resubmission.

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

      ok thanks,

      Due to unnecessary resubmission i lost around 280 points today.

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

        I wouldn't consider it unnecessary if you personally had doubts about your solution. If you couldn't assure yourself that it would pass, then it would have been risky not to resubmit, since you would have lost a lot more points if it failed. Better to resubmit early than to wait until it gets hacked, or worse, until it fails system testing.

        I think the real lesson is that you should really examine your code carefully before the first submission, and that if you have some doubts later on, you should think carefully about whether such concerns could actually prevent acceptance. If you are unable to clear such concerns, then I think you should resubmit without regrets, even if you realize after the contest that it was okay. Hindsight is 20/20, after all.

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

    According to Codeforces Contest Rules

    If a contestant submits several times a problem's solution that passes all pretests, then the last solution is considered as the contestant's verified solution for this problem. All other solutions will be considered as unsuccessful attempts.

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

    The rules are that a resubmission will incur the -50 penalty, regardless of whether the former would have passed system test or not. It's unfortunate, but that's how it is.

    While there is no way to know for sure whether your first submission would have passed the system test now, I think int should be perfectly fine, because the largest square that needs to be considered is at most $$$2 (n - 1)$$$, which is well within int limits (unless you did something really crazy in your solution).

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4 years ago, hide # |
← Rev. 2  
Vote: I like it -10 Vote: I do not like it

Amazing contest!

I am very happy because I solved 3 problems (A, B and C), unfortunately I solved them too late with 2 WA on pretest 2 verdicts. Does anybody know some real tips on how to solve problems faster and with less wrong submissions?

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

Great problems. (Specially D) How to solve D? Did anyone get AC using randomized algorithms, it would be great if someone could share some insights.

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

    hint: you can determine the winner of the four using only 2 questions

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    4 years ago, hide # ^ |
    ← Rev. 2  
    Vote: I like it +6 Vote: I do not like it

    When you query four consecutive people, it only takes two queries to figure out who the winner is out of the four people. Maintain the array of winners from the last iteration, repeat the two queries per four people, until you have either two people, or a single person, then we know the winner. The number of queries used is the sum $$$2^{n-1} + 2^{n-3} + ... + 2^1 (or~2^0)$$$ and it is easy to see that this is below the query limits

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4 years ago, hide # |
← Rev. 2  
Vote: I like it 0 Vote: I do not like it

what is the observation for b? time is very tight for c can an o(n*k) solution pass main tests? where k is number of perfect numbers less than 2 * 1e5?

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4 years ago, hide # |
 
Vote: I like it +8 Vote: I do not like it

For problem D the time limit was so tight using java it passed with C++ but cost me 20 minutes to debug in C++ since I don't use it much, couldn't you make a larger time limit for java? other than that the problems were really good.

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4 years ago, hide # |
← Rev. 2  
Vote: I like it +14 Vote: I do not like it

As a Candidate master, I even can not solve C, and I got the worst standing since I sign in Codeforces... sad

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    4 years ago, hide # ^ |
     
    Vote: I like it -28 Vote: I do not like it

    C is worst problem in long time... we have to see simple but completly random stupid observation. Like christmas riddle for pre schoolers.

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

      The fact that there always exists a square between $$$[n, 2n]$$$ is not something I would consider to be stupid at all... In fact, this is not only easily observed from thinking of small examples, but it's also easy to prove.

      Similarly, the observation that $$$(a + b) = (a + i + b - i)$$$ is extremely trivial.

      Neither of these are stupid. It does require some mathematical maturity to realize that these two observations would lead to an exact solution, but there is no randomness or stupidity involved here.

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

      It's not a stupid observation. You can do strong induction and claim that if you can build array using numbers (0,i), for each 0<=i<=n, it's also possible to do so with numbers (0,n+1). This is due to the fact that there's at least one perfect square between i and 2*i. Now you have construction using the given prime for finding (n+1)-th element, and thus can fill the suffix (i,n+1) in increasing fashion, and due to induction hypothesis we can build the remaining part.

      It's a cute problem imo.

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

      I think both C&D are tricky, agree with that the observation of C is quite random. Again, sadly I didn't pass any of them... This time I may lose more than 150 rating ... :(

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

    As a Specialist, I even can not solve B, and i got the worst standing since i sign in Codeforces... sad

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

      :( I am too sad and I can not fall asleep. This contest exposed my problems, I'm an idiot in number theory

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

        You are very good sir ! I am an idiot in everything ;((((((. I think I will quit cp and hire at foodpanda after i get my bike fixed

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

First time I solved pD on div.2!

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4 years ago, hide # |
← Rev. 2  
Vote: I like it +11 Vote: I do not like it

Has anyone Accepted the problem-D using Java? I have tried reusing array[1 << 15] but the TL seems too tight.

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

B is very similar to a USACO problem

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

    I solved Air Cownditioning for fun a few days back and was able to solve it using that.

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4 years ago, hide # |
 
Vote: I like it +36 Vote: I do not like it

Sad for Python users — D was very difficult to complete within the time limit.

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

what's wrong with this approach for D?
get winner of 1-2 pair, now we covered participants up to covered=2, then go iterating over next participants from covered+1 to covered*2. if someone won more matches then it's a winner for range from 1 to covered*2. now change covered=covered*2 and repeat until end.
somehow I get WA

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

Is the idea for $$$E$$$ correct? I got WA2.

Create graph with $$$n$$$ vertices. Iterate over all $$$i \lt j$$$ and if $$$a[i][j] \lt a[j][i]$$$ add edge with $$$xor = 1$$$, meaning, we have to swap either $$$i$$$ or $$$j$$$ cross, if $$$a[i][j] \gt a[j][i]$$$ add edge with $$$xor = 0$$$, meaning, we have to either swap both $$$i$$$ and $$$j$$$ cross, or not, if $$$a[i][j] = a[j][i]$$$, don't add edge.

All edges have time of appearing. We have to satisfy the greatest prefix of such edges. Let's do binary search. We fixed subgraph with times $$$ \lt x$$$. We have to set to all vertices value 0 or 1, to satisfy all edges' xor. First set all vertices undefined. Iterate over vertices, if we see undefined, set to it any value and do dfs. Dfs only walks on available edges, if it sees undefined neighbour vertice, it sets correct value to it and goes to it. If it sees neighbour vertices, such that is doesn't satisfy edge's xor, then we can't satisfy this prefix of edges.

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

    Furthermore, seems I solved $$$D$$$ in $$$\frac{1}{3} \cdot 2^n$$$ queries. What is expected to do with 2 times more queries?

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

    Notice that you have to repeatedly do this process as after the greatest prefix and the first "unsatisfiable restriction" , we may still have restrictions that can be satisfied, so we would probably get a $$$O(mn) = O(n^3)$$$ solution

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

    The optimal solution is not neccessaty formed as a prefix.It means you can choose "not add edge i" and "add edge (i+k)" at the same time.

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4 years ago, hide # |
 
Vote: I like it +7 Vote: I do not like it

167298607 why this got TLE ????

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4 years ago, hide # |
 
Vote: I like it +42 Vote: I do not like it

I have probably used all of my luck for the next few months: Screenshot-2022-08-06-093710

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

I loved this contest!

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4 years ago, hide # |
← Rev. 2  
Vote: I like it +8 Vote: I do not like it

D was reallllly difficult to complete within the time limit for Python user!!!!!!!!!!!!!!! It's unfair!!!, please rejudge it!!!

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

    You know that the input and output in Python is quiet slow, even my submission is the same as the tutorial , also got TLE, really unfair!!

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

    sad, always python users have to wait to increase to the next tier because of the RLE or TLE questions which happens so frequently. It's always absurd optimisations to made it pass. My friend has been waiting one year for expert and he couldn't hit it again today because he TLEd on D and he got a -60 instead, he's 5 stars on codechef (almost 6 stars) and 2450 on LC

    I really feel you

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

      Thanks for understanding!! Always Python user are not be considered in the CP. in the future if I have the chance to hold a contest, I must set different time limit restrict for different programming language, hope one day it can make true

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

        In leetcode yesterday I made 8 different TLE on O(26*10^5) algorithms.Top down was giving MLE, bottom up with O(n) space was giving TLE as well,I was so surprised that I submitted the same code multiple times, then finally optimised it to O(26) space and it passed

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

          Once, i took part in the LeetCode weekly contest and met the same thing, really sad

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4 years ago, hide # |
 
Vote: I like it +9 Vote: I do not like it
  • For D Problem, I applied the same logic as mentioned in the editorial, but i am getting Wrong Answer.
  • I have tried my best but I am not able to figure out the mistake.
  • Can someone please help me to figure out the mistake? 167302006
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4 years ago, hide # |
 
Vote: I like it +5 Vote: I do not like it

167304705 why is this got TLE???

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

what is wrong in my approach for problem B https://mirror.codeforces.com/contest/1713/submission/167293528 Approach inserted all the elements of the array into the set if the size of set is equal to the size of array this means that the all the permutations have cost greater or equal to array therefore YES

if not then i m checking the position of duplicate elements if all duplicate elements are adjacent this means the permutations of it has greater cost so the answer is yes otherwise the answer is no.

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

    The NO-instances are not characterized by duplicate elements. But rather, they are characterized by the transition from decreasing to increasing. For example, the array [2, 1, 3] should output "NO" (it requires 4 operations, whereas [1, 2, 3] only requires 3 operations), even though there are no duplicates.

    The issue with decreasing->increasing is that the two sides cannot be decremented at the same time once the center becomes 0, whereas a sorted/reverse-sorted/increasing->decreasing array can ensure that every operation hits all non-zero elements at once.

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

Thanks for the great contest :) !!!! Really loved all the problems and especially problem D And a great rating increment :)

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4 years ago, hide # |
 
Vote: I like it +8 Vote: I do not like it

Great problems. To be fair, I raged pretty hard about the time limit being way too tight on D (or there being way too many queries for an interactive problem), but other than that, great set :)

Screencast and solutions to A-D will be available on my youtube channel as soon as youtube finishes process it.

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

the pretest of B is shit!

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

In problem B there should have been a pretest where you needed to use long long int!

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    4 years ago, hide # ^ |
     
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    What? You don't need to use long long int?

    I easy solved it just with ints: 167240602.

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      4 years ago, hide # ^ |
       
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      There is a solution where participants want to check if $$$\sum(max(0, a[i] - a[i-1]))$$$ is equal to $$$max(a[i])$$$. In fact, this is also a correct solution but it may require a $$$sum$$$ variable to store big number which can only fit $$$long \ long \ int$$$.

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4 years ago, hide # |
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Balanced contest! Edit: I got expert back!

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In problem D, if n=6, can we make 43 queries?

As, (2 ^ 7)/3 is around 42.67

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can someone tell me what is wrong with my B solution: https://mirror.codeforces.com/contest/1713/submission/167256508 . i've been trying to figure it out for the past 3 hours :)

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Vote: I like it +16 Vote: I do not like it

The best contest of 2022! Orz thanhchauns2

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Interesting E. I come up with it just after the contest finished:(

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Any particular reason for C being of 1750 points?, I think it was not that hard and could have been of 1250 or 1500 points.

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Really good questions. Thanks for the contest

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Use greedy strategy, problem D could be solved within 2^(n-1) queries?

this submission https://mirror.codeforces.com/contest/1713/submission/167314546 assert queries * 2 less or equal 2^n

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In problem D, In the test case given, can't 2 win the contest?

1 2 3 4 5 6 7 8

2 4 5 7

2 7

2

The final win array becomes = [0, 3, 0, 1, 1, 0, 2, 0] Can someone explain this.... any point I'm missing?

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    4 years ago, hide # ^ |
     
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    Yes, it can. But it gives the right answer 7, so it is accepted, even though its logic may be nonsense at all, i.e. the tester only cares about the final answer, rather than the logic behind.

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wish there were stronger pretests :c

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Expected Brood War themed problems.

thanhchauns2 didn't deliver.

As a consequence, I had a terrible performance. Lowest rating in almost 2 years :(

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    4 years ago, hide # ^ |
     
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    Unfortunately, all problems with Brood War theme is either saved or removed. The set is entirely different from the first. I feel so sorry about this.

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4 years ago, hide # |
 
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Shit interactive problem

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