omggg's blog

Re-rooting Tree DP?
By omggg, history, 5 years ago, In English

I am facing issues in questions in which Given is a tree with no fixed root, n<=1e5 . I can simply do the question if i traverse the tree for each node as root (n^2) but it gives TLE. I know i have to apply DP and rerooting concept.

Can anyone provide me a blog for this particular topic? Or some handful questions on that to learn with the topic? (my rating:1600)

Any help is appreciated. Thanks

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5 years ago, # |
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Subtree can be specified not by a (root; subtree root) pair (there are $$$O(n^2)$$$ of them), but by (subtree root; edge to subtree's parent, possibly none) pair. And there are only $$$O(n)$$$ of them.

So instead of calculating parent[v] and running dfs(int v), you can run dfs(int v, int parent = -1) and cache all results for a specific vertex in a hash map.

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5 years ago, # |
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May be this one can help.

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5 years ago, # |
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Hi, I know one such question involving the re-rooting concept. https://mirror.codeforces.com/contest/1092/problem/F

I hope it helps!

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5 years ago, # |
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1324F - Maximum White Subtree

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5 years ago, # |
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This comment has problems that can be solved using rerooting idea comment

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