Vovuh's blog

By Vovuh, history, 4 months ago, In English,

1176A - Divide it!

Idea: Vovuh

Tutorial
Solution

1176B - Merge it!

Idea: MikeMirzayanov

Tutorial
Solution

1176C - Lose it!

Idea: Vovuh

Tutorial
Solution

1176D - Recover it!

Idea: MikeMirzayanov

Tutorial
Solution

1176E - Cover it!

Idea: MikeMirzayanov

Tutorial
Solution

1176F - Destroy it!

Idea: BledDest

Tutorial
Solution
 
 
 
 
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4 months ago, # |
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Problem E was little tricky, learned a lot. Thanks a lot for great contest!!

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    4 months ago, # ^ |
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    can u explain problem E? I haven't understood the tutorial. thanks in advance!!

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      4 months ago, # ^ |
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      Maintain a adjacency list, and then arrange the vertices by number of other vertices it is connected to(v[current_vertex].size) in a multiset or priority_queue (descending order). Pop to the priority_queue and mark the vertex as visited and push it in ans_vector. Iterate all the vertices to which it is connected as also mark them true but dont put these vertices in ans. (Because you need to maintain different colour). Do this until all vertices are visited.

      Ex: For a tree with 3 vertices and 2 edge: 1-2 and 2-3. Vertex 2 will have max size(2), push it in ans_vector and also mark 1,2 and 3 as true.

      Now you have an answer vector. If the size if less than equal n/2 print it otherwise print those which not not in ans_vector . This is because the set of vertices which are in ans_vector and which aren't both satisfies the condition of given problem (expect for size one)

      My submission: https://codeforces.com/contest/1176/submission/55381513 Though i was not able to do it in contest.

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        4 months ago, # ^ |
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        thanks!!

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        4 months ago, # ^ |
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        If someone is trying this approach, try this case. You will understand why pure greedy approach won't work.

        1
        9 8
        1 3
        2 3
        3 4
        4 5
        4 6
        9 7
        4 9
        9 8

        Thats why we have to use either the found set or compliment of it.

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          3 months ago, # ^ |
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          Thanks for your testcase. I thought greedy was possible but I was wrong.

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      4 months ago, # ^ |
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      Alternately, you can try the approach given in tutorial.

      Use BFS. Since BFS transverses the graph layer-wise, you can 'color' the alternate layers with color 0 and 1.

      Ex:For a tree with 3 vertices and 2 edge: 1-2 and 2-3. Color vertices 1 and 3 with color 0 and 2 with color 1. You would get two answers, the vertices those on even layers and the other on odd. Print the one with minimum size.

      This is a more cool approach :)

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4 months ago, # |
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Anyone having solution of E using DSU? or give some idea?

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4 months ago, # |
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"you can just simulate the process, performing actions from third to first."__

why in that order ?

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    4 months ago, # ^ |
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    Every time you perform operations 2 and 3, you multiply the number with 2 or 4. Therefore, each application of 2 and 3 will make the number divisible by 2, thus forcing you to perform operation 1. If you perform 3 and 2 first until you cannot anymore, then the only possible operation you need to perform is 1.

    Of course, you can perform the operations in any order you want (eg. 1, 3, 2, 1, 2, etc) as long as you can still perform one.

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      4 months ago, # ^ |
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      why 3*cnt5 not 4*cnt5 in problem A? please answer.

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        4 months ago, # ^ |
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        for each power of 5 you have to do:

        • apply once operation 3 (/5, * 4)

        • apply twice operation 1 (divide twice by 2 since you multiplied with 4 before)

        so 3 operations

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4 months ago, # |
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How would you do problem C with DP and binary search? Thanks!

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4 months ago, # |
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anyone, please explain the problem D in Graph (dfs and similar) approach. Thanks

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How to solve problem F with DP ?? Thanks!

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4 months ago, # |
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Won't the solution to the problem D have a worst time complexity of O(n^2) ?

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    4 months ago, # ^ |
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    55473892

    I guess my submission is clear it has some comments if you did not get the idea I can explain it a little bit more here

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4 months ago, # |
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Can someone please explain problem F I am not able to understand the editorial solution. Thanks in advance.

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4 months ago, # |
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is the graph is bipartite?

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4 months ago, # |
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Hello. I'm interested. Do someone implement some algorithms and data structures from scratch during competition or they have been implemented? I mean sieve, for example, in task D.

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    4 months ago, # ^ |
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    in my opinion,one should be familiar with sieve through practice so that one can easily implement it within a minute during contest.

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4 months ago, # |
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why 3*cnt5 not 4*cnt5 in problem A? please answer.

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    4 months ago, # ^ |
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    you see. Here's three steps to execute: 1.perform the third operation: n = n * 4 / 5 2.perform the first operation: n = n / 2 3.perform the first operation: n = n / 2 after theses three steps the result you got is the n you type divided by 5. so it's 3 * cnt5.

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4 months ago, # |
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Can anybody explain me problem F, the problem say that "you choose the cards and the exact order they are played", I think that if we have 3 cards 1, 2 ,3, we have to choose card 1, 2 and 3, not 3, 2 ,1 or 1, 3, 2, ... But I read that the solution has sorted the array and chosen maximum if we can, I think in some case, it will wrong, because we have neglected that we have to choose the cards in order they are played. Sorry about bad english

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4 months ago, # |
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Hey! can someone please tell me what is wrong with my code? http://codeforces.com/contest/1176/submission/55369953

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4 months ago, # |
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Can someone tell me why this python code is giving WA while its c++ version is giving correct result: PROBLEM A: ~~~~~ Your code here... t = int(input()) while (t > 0): t = t — 1 n = int(input()) count = 0 while (n % 5 == 0): count = count + 1 n = 4 * n / 5 while (n % 3 == 0): count = count + 1 n = 2 * n / 3 while (n % 2 == 0): count = count + 1 n = n / 2 if (n == 1): print(count) else: print("-1") n = 0 ~~~~~

link: https://codeforces.com/contest/1176/submission/55432686

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4 months ago, # |
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Can anyone tell me why it is not accepting for problem E [](https://codeforces.com/contest/1176/submission/55372927)

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4 months ago, # |
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@MikeMirzayanov Can you please tell me why this approach for problem E fails?

Basically I am greedily selecting vertices if none of their neighbours have been selected. At the end ,if the selected set has more than ceil(n/2) vertices then i print the complementary set

I feel this should work because one of the sets ie the original or the complementary set should have size less than or equal to ceil(n/2) and no two vertices in the original set have edges between them , so they are connected to some vertex in the complementary set (since the graph is connected) . So the complementary set is also a possible answer.

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    4 months ago, # ^ |
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    I just now realized the mistake in my code. I should have checked for the floor(n/2) instead of ceil of n/2. Thanks anyways

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4 months ago, # |
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why 5 4 3 is not a valid answer for D for below test case-- 3 3 5 2 3 2 4

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    4 months ago, # ^ |
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    In problem statement, "if a[i] is a prime number, then one integer p[a[i]] is appended to array b" if a[i]=5 and here 5 is prime number so 5th no. prime number is 11 which must appended in array b.But 11 is not in given array b that's why 5 4 3 is not a valid answer.

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4 months ago, # |
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For E,I tried "Graph coloring" with G and R such that neighbouring nodes have different colors,through BFS,now count the number of Green and Red,which ever is less than n/2,print that.

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4 months ago, # |
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PROBLEM C CAN SOMEBODY TELL ME WHY I M GETTING TIME OUT IN #TC 9.Ii know m algo is not efficient but i m not able to get editorial pls help my solution is 55554895 55554895

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4 months ago, # |
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Can someone please explain Problem D in detial.

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4 months ago, # |
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Ok I'm a noob, I absolutely suck, and I can't program for my life. Can someone explain problem B in detail to me -_- sorry about it

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    4 months ago, # ^ |
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    Look, when we divide a number by 3, the only possible remainders are 0, 1, and 2.

    Now consider an array: (3, 4, 7, 1, 1, 2, 4, 11, 12)

          remainders --> (0, 1, 1, 1, 1, 2, 1, 2, 0)
    

    For this array, let cnt[0] denote total numbers which give remainder 0 when divided by 3, similarly cnt[1] for remainder 1 and cnt[2] for remainder 2.

    So, cnt[0] = 2, cnt[1] = 5, cnt[2] = 2

    Now the idea is to add all the numbers which have remainder 1 & 2, so that when they are added the new element is divisible by 3. Since there are only 2 elements with remainder 2 (4 and 11) in this array, we add these to 2 elements with remainder 1 (say, 4 and 7).

    New array is: (3, 6, 18, 1, 1, 4, 12)

    Now, cnt[0] = 4, cnt[1] = 3, cnt[2] = 0

    After this operation the number of elements that are divisible by 3 in this array (cnt[0]) has increased. For the remaining elements (cnt[1]), we add these together. because 1 + 1 + 1 = 3.

    Finally the array becomes: (3, 6, 18, 6, 12) which has the max number of elements divisible by 3

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    4 months ago, # ^ |
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    Tell me if u don't get it somewhere I will try to explain more.

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      4 months ago, # ^ |
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      This makes sense to me now. Thank you so much.

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4 months ago, # |
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(Maybe my English is poor)

My idea is similar to the tutorial. This codedoes not exceed O(n), isn't it? But it really timed out. Why is this?

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3 months ago, # |
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what is wrong in this solution of problem D( Recover it!)?

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    3 weeks ago, # ^ |
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    hey i have a WA on same test as u had. Can u tell me what was the problem? or what was that tricky test?

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2 months ago, # |
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For Problem E, apparently just finding a bipartite coloring of the graph will suffice. That is what I did and got AC. My submission (in java) is here: https://codeforces.com/contest/1176/submission/58155189