Could not find any discussion on problems, so I am posting here.
How to solve B and L?
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Codeforces (c) Copyright 2010-2024 Михаил Мирзаянов
Соревнования по программированию 2.0
Время на сервере: 05.12.2024 05:12:00 (i1).
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B:
An interval [L,R] of 1s (0 at the L-1 and R+1) is possible if and only if, the number of fallen boxes in this interval equals R-L+1 and also there's no boxes in L-1 and R+1. So we can use a dp solution. After this observation and O(N3) dp solution it's easy to decrease complexity.
O(N3) Solution: https://ideone.com/fM5nVp
O(N2) Solution: https://ideone.com/7pZh4R
O(N * log(N)) Solution: https://ideone.com/GHDnd1
O(N) Solution: https://ideone.com/L9o6dz
L: Answer is always 2 or 3, I think it's easy to come up with a solution to calculate the number of ways after that.
Thanks a lot. In L, what is the proof that answer will always be 2 or 3?
The number of the edges is 4(n - 1) and there are only n nodes. So there must be a node with degree ≤ 3.
In L, can you please give me some hints on how to calculate the number of ways?
The solution after being aware of that the answer is 2 or 3, can be enumerate the cutting edge on a tree, and calculate which of the edges should be cut on the other tree. A trick solution is to use LCA and cal sum of subtree when using DFS. Another solution is just use dsu on tree, then u can easily calculate the answer.