For technical reasons, some programming languages (Kotlin, C #) will not be available during round 877. ×

zoomswk's blog

By zoomswk, 7 months ago, In English,

Hi, everyone!

Unfortunately, the round was moved two hours later. Sorry for the inconvenience.

It is my honor to announce that the Codeforces Round #408 rated for the second division is going to take place tomorrow at 16:35 UTC. As usual, participants from the first division will be able to participate out of competition.

As the author of this contest, I (zoomswk) would like to thank poomrokc, nisaruj, and Phoom for testing the problems, KAN and netman for their help in contest preparation, and MikeMirzayanov for the awesome Codeforces and Polygon platforms. Cute graphics in this round are designed by my friend Chonphuech Sripongtanakul, so thanks to her also!

In this round, you will be given 6 problems and 2 hours to solve them. Zane the Wizard, along with his puppy and his crush, will be asking for your help. It is advised that you read all problems and read them carefully.

As per tradition, the scoring distribution will be announced later.

I hope you will enjoy the problems.

Good luck! :D

UPD: The scoring distribution is 500-750-1000-1500-2000-2500.

UPD: The round has ended. Thanks everyone for participating. I'm deeply sorry that the problems turn out to be way harder than I expected. Please stay tuned for the editorial. T_T

UPD: The system testing is complete. Congratulations to the winners!

Div. 2 Winners

  1. Wissenschaft

  2. newcolas

  3. ckw1140

  4. VAVAvile

  5. pantelija

  6. Harmonica

  7. mateuszdanowski

  8. Mohammad_kilani

  9. Al2K

  10. LovelyPLY

Div. 1 Winners

  1. fanache99

  2. HellKitsune

  3. unused

  4. KrK

  5. petrescu

I sincerely apologize that slow input/output methods caused so many TLEs. Unfortunately, it is not possible to rejudge the submissions nor increase the time limit at this point. This is my fault, and I'm terribly sorry. I was not aware of this because my solutions run in < 500 ms of time limit in all problems. I hope you understand.

The editorial will be published in a few minutes, and I'll update this post when it's available.

UPD: The editorial is ready!

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

Good luck to you too! I wish your tests to be strong and your problems to be interesting to read, understandable and challenging! :)

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    7 months ago, # ^ |
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    Thanks! I'm trying my best to make all that happen. ;)

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7 months ago, # |
Rev. 2   Vote: I like it +55 Vote: I do not like it

It's gonna be my first round after breakup :D

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7 months ago, # |
  Vote: I like it +23 Vote: I do not like it

Zane is The Wizard but still can't make his crush into his girlfriend. I feel it.

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    7 months ago, # ^ |
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    Don't worry, you'll get to help him in the contest. XD

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      7 months ago, # ^ |
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      Why do i get the feeling that he is going to ask our help to make his crush into girlfriend.

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        6 months ago, # ^ |
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        LOOK AT THIS HANDSOME MAN

        HOW DARE YOU SUGGEST HE NEEDS YOUR HELP IN GETTING A GIRLFRIEND

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7 months ago, # |
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I can't register to unofficial participation.

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7 months ago, # |
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Div 1 Users are not able to register.

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6 months ago, # |
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How I wish there to be earlier contests since i'm in UTC+8 area.

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6 months ago, # |
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Delaying it 2 hours makes me able to do this round, thank you! :D

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6 months ago, # |
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Finally zoomswk is writing problems for CF! I heard your problems were really good from some other Triam Udom students in POSN camp 2. Looking forward to participating today!

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    6 months ago, # ^ |
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    Thanks! Hope you'll like my problems too.

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6 months ago, # |
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Thanks for the delay zoomswk, I would have missed the round. Sorry for them who had suitable time.

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6 months ago, # |
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Pleasant unexpected delay. Now I won't have to sacrifice my dinner this time :)

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6 months ago, # |
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Too bad, I can't do the round now :(

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6 months ago, # |
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Rating prediction here

Extensions:

Have fun & high rating:)

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    6 months ago, # ^ |
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    I am unable to see the predictions.

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      6 months ago, # ^ |
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      When contest start, prediction will work.

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    6 months ago, # ^ |
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    Thank you as always ! :D

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6 months ago, # |
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rip UTC+8

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6 months ago, # |
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div 2 after long days. ;(

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6 months ago, # |
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1900 a rating everyone must get a lving![contest:408]

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6 months ago, # |
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It's not a good time for Chinese now, so sad. :(

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6 months ago, # |
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Thank you zoomswk for giving us the opportunity to help someone.

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6 months ago, # |
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We are not able to help our self with our crush. Lets see if we can help the wizard! XD

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6 months ago, # |
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good luck

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6 months ago, # |
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Such an interesting scoring distribution :D

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6 months ago, # |
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After a long time, a 750 problem.

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6 months ago, # |
Rev. 2   Vote: I like it -12 Vote: I do not like it

after locking can't see soln of others plz solve this problem as soon as possible

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6 months ago, # |
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in problem b , for eg: 4 to 6 swap takes place then is it sure next swap will be from 6 only to somewhere else ?

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6 months ago, # |
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I can't understand how I managed not to read the only word in bold in the whole statement of C and spending the whole contest solving a different problem...

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6 months ago, # |
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Look at first and second page of standings :D

remove them pls before rating update ;D

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6 months ago, # |
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now it looks like 20+ people sit together and code together and submit at the same time :P

why this guy so boring?

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6 months ago, # |
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These Bots!

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6 months ago, # |
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Codeforces Round #408 (Div. 1,5) :)

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6 months ago, # |
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is there any better order for F that O(n*sqrt) ?

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    6 months ago, # ^ |
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    My solution works in O((n + m)logn). Please stay tuned for the editorial.

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6 months ago, # |
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Am I right that answer for D is always K - 1 (may be someone even can proof that)?
UPD Regarding to comment below that's actually number_of_cities_with_police  - 1.
If so, that will explain why we had to print sertificate for an answer.

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    6 months ago, # ^ |
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    I guess because the tree is arranged according to the law. So the question is actually how to destroy some roads in order to get number_of_cities_with_police connected components. The best way is to have one police station in every connected component. And because in the tree every city is at most d units away, it is possible to keep this property in partition at the end. I hope you understand :)

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6 months ago, # |
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Including multiple police stations in the same city in D was evil. Cost me 2 WAs. :(
Unbalanced problem set but nice problems! :D

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6 months ago, # |
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Can someone please tell How to solve C ???? I spent all time on it ,yet no success :(

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6 months ago, # |
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WTF is C. C is too difficult

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Someone tell me how to solve Problem D & E please, I can't wait for the editorial xD

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    6 months ago, # ^ |
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    In problem D we can run a single bfs from every police station. Then for every city we have its distance to its closest police station as well as the number of different police stations that has its distance equal to the shortest distance. Run a single dfs from vertex 1. For each vertex, let's consider its children. If a children has abs(dis[v]-dis[u])!=1 then we will cut that edge. If dis[v] == dis[u] — 1 then we will count how many vertex like that and we will erase (cnt-1) of them because we only need one of them to maintain the distance. And for (dis[v] == dis[u] + 1), as we already have the number of ways to get to this vertex, we will delete this edge if way[v] > 1 and then way[v]--. That's all!

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    6 months ago, # ^ |
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    D is a simultaneously BFS from every city with police. Mark each connected city with nubmer of a source police city, then go through edges and if a "police number" on the left != one on the right, then the edge can be removed. You even don't need the d parameter.

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      6 months ago, # ^ |
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      I did mostly the same, but only marked cities as "connected" without the number of the source and also marked "used" edges. And if I come upon an unused edge that goes to an already connected city then it can be removed.

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6 months ago, # |
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The girl's cheating algorithm in E is so complex, almost as if you have experience with it :P

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For problem B, what is the expected output for this input?

4 1 3 4 3 4 1 3 3 2

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    2

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    My program prints 2.

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      Shouldn't it be 3? Confused.

      When 3 and 4 are swapped, the hole should be underneath cup 3. Again when 1 and 3 are swapped, the bone gets into the hole. Now when 3 and 2 are swapped there should be no change. Did I understand the question incorrectly? :/

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        6 months ago, # ^ |
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        Ahh, positions of the cups, got it!

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        6 months ago, # ^ |
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        You input n m k then you input the places of the holes.

        In your test case the hole is in number 4.

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        6 months ago, # ^ |
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        the hole can not be swapped its constant during swaps

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        6 months ago, # ^ |
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        The holes don't move when swapping cups.

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Can we use greedy to solve the problem D ?

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when will he post the editorial?

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6 months ago, # |
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Can anyone explain me B? I dont get it completely... more like i thought i knew what was going on there, but i did 9 fails and i dont even know why Was he also changing positions of the holes?

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6 months ago, # |
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Can someone provide hack case for B?

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    6 months ago, # ^ |
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    2 1 1 1 1 2

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    6 months ago, # ^ |
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    i hack using this : 2 1 1 1 1 2

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    6 months ago, # ^ |
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    2 1 1 1 1 2

    the answer is 1

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    6 months ago, # ^ |
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    TL hack with 10^6 numbers. OK if solution hasn't scanf/sync_with_stdio.

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      6 months ago, # ^ |
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      Wow, absolutely forgot about it:(

      All the contest I couldnt find any other narrow places in B besides the cases above

      I think TL was the main reason for so mane hacks

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6 months ago, # |
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Is problem C just modified BFS starting in the vertex with max guard and max degree, or is there something more? (sorry for my English if it's bad)

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    6 months ago, # ^ |
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    (wrong solution)

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      6 months ago, # ^ |
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      No, this is wrong.
      Take the case:
      6
      3 3 3 3 3 3
      1 2
      1 3
      1 4
      1 5
      1 6

      Answer would be m+1(4) for this.

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        6 months ago, # ^ |
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        Indeed, thank you for pointing this out.

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        6 months ago, # ^ |
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        What is the approach for C?

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          6 months ago, # ^ |
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          Let there be a set of nodes which have equal and maximum weight.
          For each node 'x' from the set,
          Final weight of x: Same as original weight
          Final weight of neighbors of x: Original weight+1
          Every other node: Original weight+2

          Do this for each x in the set and calculate the minimum maximum weight.

          EDIT: I got a WA. The reason is that we have to consider every node rather than just the max nodes.(Imagine case where all max nodes are connected to a single smaller node)

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            6 months ago, # ^ |
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            Shouldn't that get TLE if all of the nodes are with the same value ?

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              6 months ago, # ^ |
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              I used multisets so I only had to iterate through first node+neighbours.
              It took 1s but TLE is still a worry.

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      6 months ago, # ^ |
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      5

      5 5 4 5 5

      3 1

      3 2

      3 4

      3 5

      Wouldn't your algorithm give 7 for this case? I think it should be 6.

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      6 months ago, # ^ |
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      "If t > 3, then the answer is m+2." I think it's incorrect. For example, there can be 10 nodes with maximum value, and one node which connects with each of this nodes.

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    6 months ago, # ^ |
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    I think starting from guard with a max strength and connected to maximum number of other max guards is correct, your algorithm will not work for graph: 10 10 10 10 1 1 1 1 ... 1 2 1 3 1 4 4 5 4 6 4 7 ...

    Your algorithm starts from 4, and we should start from 1.

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-8 in B because i forgot ios::sync_with_stdio(false) D:

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it was a big leap from b to C hahaha

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  Vote: I like it +65 Vote: I do not like it

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    6 months ago, # ^ |
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    Each Id belongs to one person I guess! but if each code are same then they will be skipped without the first one. But he should get punished -_-

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6 months ago, # |
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Hi, Could someone help me out with understanding an optimal solution for D ?

Thanks!

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    6 months ago, # ^ |
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    I think we can separate the tree in forest, where the root of each tree has a police office in it. This way the answer will always be P - 1

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      6 months ago, # ^ |
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      careful with several police stations on same node:)

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S**t :( Failed to notice that multiple police can exist in one city :'( maybe that's the reason for getting wa on pretest 6

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    6 months ago, # ^ |
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    I don't think it is. I was failing on 6 for the longest time too. What does your solution output for this inout:

    5 2 2
    1 4
    1 2
    2 3
    4 3
    3 5
    

    I believe the correct output is

    1
    2
    

    (EDIT: Fixed it from 5 2 3 to 5 2 2)

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      6 months ago, # ^ |
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      1 3 And I believe that's a correct output :(

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        6 months ago, # ^ |
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        Sorry, the I actually gave you the wrong input, the first line should be 5 2 2

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        6 months ago, # ^ |
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        1 3 isn't correct btw, it should be 1 2

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    6 months ago, # ^ |
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    Damn. I failed to notice that too. My program was based on the assumption that there's only one police in one city ;-;

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I believe that it is the same guy who have the ranks between 3, 30 :D

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That feeling when you realized problem A minor bug... in the last 30 seconds

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The problem set was really very nice. I have enjoyed all the problems. Thanks for the round. Hope to get such good problems from you again.

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Submitted D with 9 seconds to spare, let the system tests be kind

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I can't forgive myself for wasting too much time to find out why was my B solution failing at test 3 ..

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    6 months ago, # ^ |
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    same here, did u find out? would u like to share with me?

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      6 months ago, # ^ |
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      I was transferring the bone in u->v , forgot that if glasses u and v are swapped then there are two possibilites u->v or u<-v. Used this, pretests passed.

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      6 months ago, # ^ |
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      Consider swapping xi and vi. I was only checking if the bone is at Xi so I move it to Vi. I wasn't checking if it is at Vi.

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    6 months ago, # ^ |
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    What was it?

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6 months ago, # |
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Fake , fake everywhere ..

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How to solve E?

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Who dis?

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How to solve B? I have wasted 1 hour and 53 minutes for damn pretest 3 on B...

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    6 months ago, # ^ |
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    1 ≤ hi ≤ 106, so you can keep them in bool array, like holes[hole] = true.

    Then check if holes[bone] = true then print bone, break else continue swapping.

    Link for submission: http://ideone.com/BvFVqI

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      6 months ago, # ^ |
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      So why didnt it pass?  Click

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        6 months ago, # ^ |
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        What if the ball is in v at the beginning?

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        6 months ago, # ^ |
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        Because of you didn't check case when v =  = ball

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          6 months ago, # ^ |
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          omfg im so fckin stupiiiiiiiiiiiiiiiiiiiiid 1:53 of hour lost for that

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        6 months ago, # ^ |
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        Ball can be in v too

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    6 months ago, # ^ |
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    I was transferring the bone in u->v , forgot that if glasses u and v are swapped then there are two possibilites u->v or u<-v. Used this, pretests passed. Take care of the current position of the bone.There might be a case in which we are swapping the glasses and neither of them has a bone.If the current pos has hole voila! answer found.Else in the end the final pos is the answer.

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    6 months ago, # ^ |
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    1) if first position contains hole , then obviously print 1 , return 0; 2) else -------> keep track of current position of bone in any variable(say boneVar) -------> Initial value of boneVal = 1(given in question)
    ----- > now iterate for k times and take values of cups to be swapped,,, suppose inputs are cup1 , cup2 for(int i = 0 ; i < k ; i++)

    for(int i = 0 ; i < k ; i++)
    { 
             cin >> cup1 >> cup2;
              if(cup1 == boneVar) boneVar = cup2;
              else if(cup2 == boneVar) boneVar = cup1;
              /// now check if the new position of bone contains hole then that is answer
             if(hole[boneVar])
             {
                  cout << boneVar;return 0;
             }
    }
    
    if(cup1 == boneVar) boneVar = cup2;
          else if(cup2 == boneVar) boneVar = cup1;
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When system testing will start?

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    6 months ago, # ^ |
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    I think it may take a while because as you can see, there are many many spam accounts in today round and moreover, their owners have used "Find and Replace" to replace names of all variables and functions which will make the CF anti-cheat accept their solutions UPD: All bots have been deleted

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From rank 3 to rank 30 , will those people be removed and ranks will be updated ???

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6 months ago, # |
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can we use this to solve E?
UPD: It works

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    6 months ago, # ^ |
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    If (p / 2) * k >= n. Obviously, It is the maximal point. Else we use dp with n * p * k states, each state has O(k) transitions.

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      6 months ago, # ^ |
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      Isn't that O(npk^2) complexity?

      I came up with a solution with O(n*p*2*k) states and constant number of transitions but didn't manage to implement it during the contest. And it also took my a while to finally accept it afterwards. It turned out to be pretty messy :/. Here is my code if you're interested.

      UPD: Ok, I missed the point. It's indeed O((n^2)k).

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        6 months ago, # ^ |
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        It is the intended complexity too. Again, timelimit is too strict :). 1000 * 1000 * 50 = 5.10^7.

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6 months ago, # |
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The cheaters are removed from the rankings, except the one who got hacked on C.

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    6 months ago, # ^ |
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    koil (save the name to remove )

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

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    6 months ago, # ^ |
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    Not for all...:( :(

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      6 months ago, # ^ |
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      It's because C question was Tricky one :)

      Contest was solely based on A and B questions for most of the people.

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6 months ago, # |
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It wasn't that hard I think.

Thanks for you effort :D

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6 months ago, # |
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Test 66.........

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6 months ago, # |
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Got to hate getting TLE for using cin instead of scanf

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6 months ago, # |
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I think there were some problems with the server during system testing. At B, many people(as have I), have got a TLE even if the complexity was linear in the numbers read.

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6 months ago, # |
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i am rotting at the newbie section

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6 months ago, # |
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TLE on B? What is this dude?

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    6 months ago, # ^ |
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    slow io

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      6 months ago, # ^ |
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      In CodeForces we shouldn't get TLE for this -_- :\
      Since our algorithm was right; we should get AC!

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6 months ago, # |
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TLE on problem B because I forgot to use fast IO 26261895

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6 months ago, # |
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Problem C TL is so tight... my solution passed pretests with 1500 something ms and so did many others. Now my solution got TLE (probably ran a test in about 2100 or so ms) while many others are getting AC with 1950+ms solutions.

Not fair in my opinion.

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6 months ago, # |
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Could , someone please tell me the reason of TLE in this code of div 2 , B? 26277647

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6 months ago, # |
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Could someone Please tell me why am I getting tle in Java in div2b problem. My code's complexity is O(K) . Here is my submission. please check Here..

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6 months ago, # |
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Time limit for problem C is too strict. Solution use multiset must pass.

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6 months ago, # |
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I got good on problems A, B and D, but after tests i only got A... Failed on TL on B and wrong answer on test 13 on D. http://codeforces.com/contest/796/submission/26274912

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6 months ago, # |
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    6 months ago, # ^ |
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    for cin, cout use these two lines in front of main: ios_base::sync_with_stdio(false); cin.tie(0);

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      6 months ago, # ^ |
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      The core of the task consist in using these "two lines in front of main: ios_base::sync_with_stdio(false); cin.tie(0);" with cin,cout?

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    6 months ago, # ^ |
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    don't use cin , use scanf

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6 months ago, # |
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Why is the system testing so slow? It seems like it is hung...

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6 months ago, # |
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WA on Test 61 on problem B!!! why ??? :( :( 26270333

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6 months ago, # |
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6 months ago, # |
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why system testing does not finishes?

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    6 months ago, # ^ |
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    Am I only one who couldn't access codeforces for about 5-7 minutes?

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6 months ago, # |
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I really hoped the purpose of Question B was more than just rejecting cin and accepting scanf -- it probably shouldn't have been a CF problem if that was really the case.

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    6 months ago, # ^ |
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    6 months ago, # ^ |
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    Really this problem shouldn't be in CF. Today I will become a pupil :'( :'(

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    6 months ago, # ^ |
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    I've solved with cin/cout (1185 ms)

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      6 months ago, # ^ |
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      You have

      ios_base::sync_with_stdio(0);
      cin.tie(0);
      

      In your code!..
      But this shouldn't be a problem in CF. Right Algo should get AC :'(

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        6 months ago, # ^ |
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        this is very common trick and it known by everyone who use cin/cout :)

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      6 months ago, # ^ |
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      What I really meant was that I didn't expect the entire purpose of the question to be testing fast I/O...

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6 months ago, # |
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In fact some codeforces round ends up in first 40-50 minutes.

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6 months ago, # |
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Slowest System Testing Ever.

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Too long testing today. Eyes are on the screen for a long time, now tired :(

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6 months ago, # |
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B and C are all about file reading optimizations. Had to switch pypy to cpython for it.

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6 months ago, # |
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It's time to eat breakfast.

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6 months ago, # |
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They should mention about the faster I/O method in the question

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6 months ago, # |
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Is it just me or recently in div 2 rounds the difficulty gap between problems is getting larger?

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6 months ago, # |
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They removed bots !

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6 months ago, # |
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Seems like they made case #66 of B only for removing solutions with cin/cout >_<

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    6 months ago, # ^ |
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    They should have made that test #1 for faster testing :D

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    6 months ago, # ^ |
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    I used cin/cout in B and got AC!

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      6 months ago, # ^ |
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      How? I used too and go TLE, with scanf it passes tests

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        6 months ago, # ^ |
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        Use this in main() to avoid TLE due to I/O!

        std::ios::sync_with_stdio(false);
        cin.tie(0);
        
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6 months ago, # |
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in C, my O(n) solution with printf/scanf got TLE :|

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    6 months ago, # ^ |
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    I managed to think nlogn. How to solve in O(n)?

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      6 months ago, # ^ |
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      One possible O(n) approach is realizing that we always start with the node that is neighbouring to the most nodes with maximum value. If the node itself has maximum value it is also counted as neighbouring itself. In the case of a tie choose a node that is maximal itself. Then hack from there.

      This works in O(n) and can be implemented quite cleanly: submission

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6 months ago, # |
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Problem B:

How is this submission displaying an additional "1" at the end? It's driving me crazy.
The output on my IDE doesn't include the additional "1" at the end.

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    6 months ago, # ^ |
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    You shall end program here if (holes[1] == 1) { cout << "1" << endl; break; // return 0; }

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      6 months ago, # ^ |
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      Still, the 'break' instruction will exit the 'while' loop to the "return 0" line.

      EDIT: removing the entire if(holes[1]==1) part from the code and running it on codeforces will still display "1" at the end.

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        6 months ago, # ^ |
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        Use this: std::ios::sync_with_stdio(false); cin.tie(0);

        And u will pass tests

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        6 months ago, # ^ |
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        The other 'break', when the ball is dropped into a hole, won't exit the 'while' loop though, since it is located inside a 'for' loop.

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        6 months ago, # ^ |
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        Well, haven't noticed while at first... But problem still is pretty the same. When you get ans = 55, there is still some data in input stream and while works until the data is gone or hole[1] == 1 case worked

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6 months ago, # |
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Can someone explain to me why I got Runtime Error on test 111 of problem C (div2)?

http://codeforces.com/contest/796/submission/26283235

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6 months ago, # |
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Yeap... Thought about using scanf(), but was too lazy...

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6 months ago, # |
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I tried dfs on the problem D,and got a wrong answer on 6.26286839 I saw the others used bfs.So dfs is a wrong way?Can someone help me ?

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6 months ago, # |
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Got runtime error test 38 Problem A http://codeforces.com/contest/796/submission/26263998

Later tried test 38 in Codeblocks, works normally and correct

WHY?

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6 months ago, # |
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Where can you see the standings (scoreboard) for the Div 1 users?

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6 months ago, # |
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DIV2-C: Got TLE in testcase 35. can anyone help me in analysing the time complexity of this submission Thank you in advance :)

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6 months ago, # |
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Hello everyone, could somebody tell me why I got AC instead of Time limit after I use scanf instead of cin for solving the problem B — Find The Bone. I'm so confused now, thanks a lot!!

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6 months ago, # |
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I got a WA on test 6, whose input is: 5 1000000000 0 1000000000 0 1000000000 1 2 2 3 3 4 4 5 and the answer is 1000000002 ; but I think the right answer is 1000000001(with hack order 3 1 5 2 4)....I need help...I can't understand why..TAT

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    6 months ago, # ^ |
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    Read the three conditions carefully. After the bank 3 is hacked, you can only choose to hack banks 2 and 4. Banks 1 and 5 cannot be hacked yet because they are not neighboring to any offline banks.

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6 months ago, # |
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I just wonder if the algorithm still work when the city is not a tree in problem D.

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    6 months ago, # ^ |
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    It will still work. The solution would be more obvious could the input be any graph, wouldn't it? XD

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      6 months ago, # ^ |
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      Got that! Thank you :) Now I see the reason that you make the country a tree haha...

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4 months ago, # |
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Is problem C in this contest updated ?

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    4 months ago, # ^ |
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    I haven't done anything with it. What's the matter? :O

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      4 months ago, # ^ |
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      I am so sorry .The wrong with me i thought this is another problem :D.Thank u for caring.