Jeel_Vaishnav's blog

By Jeel_Vaishnav, history, 11 months ago, In English,

I hope you guys enjoyed the contest and we hope to host another one soon :)

With that said, here are the tutorials:

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Author: Ashishgup

C++ Code: 46095081
Java Code: 46095332

Tutorial is loading...

Author: Jeel_Vaishnav

C++ Code: 46095083
Java Code: 46095337

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Authors: Jeel_Vaishnav, Ashishgup

C++ Code: 46095097
Java Code: 46095342

Tutorial is loading...

Authors: Jeel_Vaishnav, Ashishgup

C++ Code: 46095154
Java Code: 46095344

Tutorial is loading...

Author: Jeel_Vaishnav

Java Code: 46095349

A nice explanation of Problem E by Kognition in the comment section: https://codeforces.com/blog/entry/63352?#comment-473028

Tutorial is loading...

Author: Ashishgup

C++ Code: 46095066
Java Code: 46095373

 
 
 
 
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11 months ago, # |
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Editorials came in so early :)

Thank you Jeel_Vaishnav and Ashishgup.

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11 months ago, # |
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Good contest,expect for the order of E and F.

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    11 months ago, # ^ |
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    Moreover,why I can't see it in contest materials.It seems that the editorials haven't attached to the contest.

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11 months ago, # |
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Did anyone solve problem E with LP solvers? If yes, what's the solver needed? I am using simplex and it gives TLE :/

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11 months ago, # |
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I love how there are Java and C++ code!

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11 months ago, # |
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Is there any proof for the author's solution of D?

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11 months ago, # |
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In F, final part may be a little bit easier. The root node is the node separated from any of two leafs by exactly h - 1 nodes (among the found 2h - 1 nodes). Still O(H2) though.

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11 months ago, # |
Rev. 2   Vote: I like it +11 Vote: I do not like it

In the final part of F, you can use std::sort like this:

std::sort(path.begin(), path.end(), 
  [&](int u, int v) {
    if (u == v) return false; 
    return Query(leftmost, u, v);
  }
);

This decreases the complexity of fixing the order to . Moreover selection algorithm will make it O(H).

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11 months ago, # |
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Auto comment: topic has been updated by Jeel_Vaishnav (previous revision, new revision, compare).

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11 months ago, # |
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What the F ;)

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11 months ago, # |
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My solution of F -
Take any 2 random nodes, say u and v. The probability that root lies in the path of u and v is 1/2.

Perform query(u, i, v), query(u, v, i) and query(i, u, v) for all i.
if query(u, i, v) = Yes, then i lies in the path of u and v
if query(u, v, i) = Yes, then i lies in the subtree of v
if query(i, u, v) = Yes, then i lies in the subtree of u

We can find subtree sizes of u and v, and thus we'll get the level of u and v. We already know the level of the root. So, by checking the number of nodes in the path of u and v, we'll get to know whether root lies in the path or not.

If root lies in the path, then we can find the order of the nodes which are in the path (as mentioned in the editorial), and thus find the root.

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11 months ago, # |
Rev. 2   Vote: I like it +1 Vote: I do not like it

Ashishgup can you plz provide explanation of your code for problem multiplicity.

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11 months ago, # |
Rev. 5   Vote: I like it 0 Vote: I do not like it

Problem — Views Matter What will be the output for Input ?

3 3
3 1 1

shouldn't the output should be " 2 " instead of " 1 " Since " 3 " blocks can manage top and right view.

Illustration

*
*
* * *

Top View — 3
Right View — 3 After Changes

*
  *
    *

Top View — 3
Right View — 3 Help me, is this correct or did I misunderstood something. Task number — 5

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    11 months ago, # ^ |
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    Ashishgup, Thanks for clearing my doubt

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    11 months ago, # ^ |
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    The answer "1" is correct because the position of any block in a stack cannot change. As per your illustration, the * in the 2nd stack should be in the 3rd row only as there's only 1 * in that stack.

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11 months ago, # |
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Excuse me, could someone please show me what I have done wrong with the WA solutions ?

Correct sol : https://codeforces.com/contest/1061/submission/46112942

WA sol : https://codeforces.com/contest/1061/submission/46097011

Summarization: In both sol, what I am trying to do is too keep the all the ends of renting time of TVs, and for the current shows, I will decide to attach it to a TV or assign it to a new one, optimally. The only difference in two solutions is the sorting of the [li, ri]. In the WA, the array was sorted in accordance to ri, while in the correct one it was sorted according to li

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11 months ago, # |
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wa in testcase 43 of problem D

https://codeforces.com/contest/1061/submission/46100789

What is wrong with the solution?

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    11 months ago, # ^ |
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    哇,

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    11 months ago, # ^ |
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    My submission also failed on test 43. For a case where it fails, read the editorial of the problem and then consider the intervals [li, ri], [lo1, ro1], [lo2, ro2] given in that. Try to draw them on paper and see how your algorithm proceeds. It assigns the same TV to [li, ri] as that of [lo1, ro1] whereas the optimal solution is to assign it the same TV as that of [lo2, ro2].

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11 months ago, # |
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dp[i][j]={ dp[i−1][j]+dp[i−1][j−1] if a[i] is a multiple of j dp[i−1][j] otherwise } Can anyone explain me this equation.

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    11 months ago, # ^ |
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    The number of good sequences of length j in the array a1, a2, ... ai is the sum of the number of good sequences of lenght j in the array a1, a2, ... ai - 1 (the number of sequences where ai is not the j-th element) plus the number of good sequences of length j - 1 in the array a1, a2, ... ai - 1 (the number of sequences where ai is the j-th element).

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      11 months ago, # ^ |
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      How does one convert this to a 1D array? The 2D approach can't be directly reduced to use 1D arrays right?

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        11 months ago, # ^ |
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        Let's maintain a 1D dp of size n. We iterate over the array and keep updating the dp. At any moment, dp[i] will be the number of ways to form subsequences of size i from the elements we iterated till now. Let the current number in the array be x. Now, we can see that the transition dp[i][j] = dp[i - 1][j] is not required here as it will be stored anyway in the dp. Only transition needed here is dp[i][j] = dp[i - 1][j] + dp[i - 1][j - 1], which is when j is a multiple of x. Also in 1D dp, the transition will become dp[j] = dp[j] + dp[j - 1]. Hence, what we can do is that we iterate over the divisors of x and update the dp accordingly.

        Note that we need to iterate the divisors in decreasing order or you may end up calculating more ways. For example, if you are iterating divisors in increasing order and x is 6, then first you will update dp[2] = dp[2] + dp[1] and then you will update dp[3] = dp[3] + dp[2]. As you can see here, you calculated more number of ways for dp[3], as you updated dp[2] before dp[3].

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          9 months ago, # ^ |
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          what is the inital value of dp ( i mean before recurring loop )

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11 months ago, # |
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I have another approach for F.

First case : k < 18

Starting from a random node (let's call it X), I basically find all of its k children and parent (if it's not the root). To do this, just find another random node which hasn't been marked yet (marking process is written below) and ask for the next node which lies on the path from our current node to it.

After finding one of its neighbor (lets call it Y), I mark all the nodes Z that ask(X, Y, Z) = "Yes", cause they are negligible.

You can easily notice that the parent of the current node is the vertice with the biggest number of vertice Z that satisfied the condition above.

If a vertice have exactly k neighbors, it's the root of the tree.

So there will be at most log(n) * n * 2 * k questions need to be asked in this case

Second case : k > 17

One thing to notice is that in this case, the depth of the tree is at most 3.

Now the idea is from a random node, find the longest simple path starting from this node and you will know exactly which node is the root on this path.

For instance, let n = 421 and k = 20 and you've found the longest path starting from node X which its length is 3 then you can be sure that the second node lying on this path is the root of the tree (sorry for poor explanation).

Now, to find the longest path starting from a node X, my solution is to get 50 other random vertices and find ask for the length from X to these nodes. One can easily see that the probability of getting one of the longest paths in this way is >= 0.5 for each random node we ask.

After getting nodes from one of the longest paths, you can easily sort this path in the correct order and find the answer.

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11 months ago, # |
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Is there any way to test problem F on my machine?

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11 months ago, # |
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I thought that the checker for F could be adversarial. Like if your algorithm is to randomly pick nodes until you get leafs where the root is their Lowest Common Ancestor, then the adversarial checker would give you neighboring nodes instead. It could defer assigning final positions to the nodes unless query answers lead to contradictions.

I think I have a algorithm that does something like  ≈ 50N queries when the checker is as adversarial as possible and the tree height is 10 (which is the max for the problem).

I start with picking two random nodes a, b and making queries (a, x, b) until I find a node in between. That's N - 2 steps. Then I find the depth of x and all nodes reachable through it's parent by fixing a, x and making queries (a, x, y) and then fixing b, x and making all queries (b, x, y). After 2N - 4 queries you know which ones of a, b are above x, the depth of x, and all nodes reachable through x:s parent. If x is the root, we will detect it, because we know it's depth.

Then the idea is to find a node z on the path from x to the root, and continue going up from z. We pick any node t in the subtree reachable from x through it's parent. Then we check what nodes are on the path between x and t in  ≤ N - 2 steps. We also check the depth of t in  ≤ 2N - 4 steps. Then we sort the path from x to t to find the highest node on it. Sorting can be quadratic, but that's fine, because the path is  ≤ 19 elements long, and the longer it is, the higher up in the tree we will get.

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    11 months ago, # ^ |
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    Very nice idea of yours. In your explanation, the sentence "I start with picking two random nodes a, b" is misleading because it makes think that your algorithm is random. You could just say "I start by picking any two nodes a,b, e.g. a=1, b=2".

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11 months ago, # |
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Are the complexities for E correct? Min-Cost-Max-Flow should take O(nm) iterations (like Edmonds-Karp), and in each iteration Bellman-Ford takes O(nm), which gives O(n2m2). In our case, since m = O(n) we would get O(n4).

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    11 months ago, # ^ |
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    It is correct. You can keep a potential value to be able to use dijkstra instead of bellman-ford to make it O(mlogm) per augmenting path. More details here: https://www.topcoder.com/community/competitive-programming/tutorials/minimum-cost-flow-part-two-algorithms/

    Just to be more clear, this algorithm runs bellman-form 1 time to get initial potentials and maintains it so that the edges have modified weights >= 0 but they still get the correct min path.

    Quick edit: there will be O(N) augmenting paths so the complexity is O(#augmenting paths * cost of finding an augmenting path)

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    11 months ago, # ^ |
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    Complexity of Min-Cost Max-Flow using Bellman Ford is O(flow·V·E). Here, maximum flow will be n and even V and E will be n and so overall complexity will be O(n3).

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      11 months ago, # ^ |
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      Oh, the Ford-Fulkerson argument (each augmenting path increases flow by at least 1). I see.

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11 months ago, # |
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I would appreciate if someone could help me understand why this submission 46126706 fails and this one gets AC 46126678. The only difference between them is in function f :(.

Spoiler
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    11 months ago, # ^ |
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    "if( f(*it, l) > x)". Probably this line, because you cannot compare after taking modulo.

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    11 months ago, # ^ |
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    Sometimes because of module function f will appear smaller than x so when you use this function for comparisons you need to stick without it.

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11 months ago, # |
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In problems like D, I always sort them by their ending time and all of them passed. Now It fails in test 43. Learned a new thing. Thanks.

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    11 months ago, # ^ |
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    Can you prove why it failed? I have encountered the same problem

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11 months ago, # |
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Another similar approach for problem F: Get two random vertex (u, v). The probability that these are two leaf from distincts subtrees of the root is 1 / 8, so the probability of fail is 7 / 8 and after 40 random pick of (u, v) the probability of fail is around 0.004. For each pick ask for all the nodes that are in the path from u to v, if these number is exactly 2 * (depth - 1) - 1 then these are two leaf from distincts subtrees and the root is there. Only left check what of these nodes are the root, this can be done with n query, because for a fixed vertex and all the others the root is the only node that are in exactly in n - 1 - (n - 1) / k paths. 46130299

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11 months ago, # |
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Pretty and short sweepline + greedy approach for problem D: 46130297

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11 months ago, # |
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I am wondering whether problem C can get accepted submission using python ..

EDIT: I can get accepted using PyPy. Does anyone know whether there is some explanation regarding python submission for contest?

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11 months ago, # |
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I don't understand the problem statement for problem B at all :(. Visual cues are not clear. The drawing doesn't explain the parameters for the problem.

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    11 months ago, # ^ |
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    The first drawing doesn't have an example which could explain the problem statement.Unless I start making assumptions which ideally I should not. I am not sure if the author is trying not to state the obvious (from his point of view), but it really helps if he does for this particular problem where you do need some visual cues.

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11 months ago, # |
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Here is my submission for C: 46183737. I get TLE, probably because my loops to calculate the divisors for every number is too slow. Is there something simple I can fix, or is my method just too slow?

Also, the author has a much more complicated way of calculating divisors. Is his way fast enough to use for pretty much every problem where it is beneficial to precalculate the divisors of numbers?

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11 months ago, # |
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Can anyone explain me why dp[j] is updated if and only if j is a divisor of a[i] ? Thank you so much

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11 months ago, # |
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As per the editorial of F, all the 60n queries are exhausted in Part 2 and 3 only. How do we afford the additional queries for the Finally part? Please let me know what am I missing here.

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10 months ago, # |
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upd
it was mistake in my idea
sorry for the trouble

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10 months ago, # |
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Can someone tell that in problem C, we want the decreasing order of divisors.I used set for this but I got TLE on testcase number 19, but when I used vector:sort, it got accepted.But both have same time complexity on an average ,or it is so that vector is more efficient than set?

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10 months ago, # |
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in B example 5: 3 3 3 1 1 i think we can remove 2 block from height 3,please help

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4 months ago, # |
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In Problem C how can we find all the divisors of a number . Isn't it hard to construct all divisors by having only prime divisors ?

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4 weeks ago, # |
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in order to find a leaf, we can act like this:

int find_leaf()
{
    u=rand(),v=rand()
    for i=1 to n 
        if query(u,v,i) 
            v=i; 
    return v; 
} 

then we can get a leaf in O(n),use n querys only.