A note on a nowcoder problem -- Centroid decomposition (点分治)

Правка en5, от Aveiro_quanyue, 2023-03-28 14:02:13

Nowcoder problem (Chinese) link

English version

First I would like to thank ShaoNianTongXue5307 for his idea!

Part 1: Problem Statement

A tree $$$T=(V, E)$$$ has $$$n$$$ vertices and $$$n-1$$$ edges, the weight of each vertex $$$i$$$ is $$$a_i$$$.

For each edge $$$e$$$, you can determine its direction, i.e., for two vertices $$$u, v$$$, there are two states: $$$u \rightarrow v$$$ and $$$v \rightarrow u$$$. There are $$$2^{n-1}$$$ states in total.

For each state $$$S$$$, we define $$$f(S)$$$ as

$$$f(S) := \sum\limits_{(u, v) \in V \times V, v\,\text{is reachable from}\,u} |a_u - a_v|$$$.

Compute the sum of $$$f(S)$$$ over all $$$2^{n-1}$$$ states $$$S$$$, modulo $$$998244353$$$.

Example:

Suppose $$$n=3$$$, and two edges connect $$$(1, 2), (2, 3)$$$, respectively. $$$a_1 = 3, a_2 = 1, a_3 = 2$$$, the answer is $$$14$$$.

Constraints: $$$2 \leq n \leq 2 \cdot 10^5$$$, $$$1 \leq a_i \leq 10^9$$$.

Part 1: Problem Statement:

Before we learn centroid decomposition, we should first know what it is good at. For a tree $$$T = (V, E)$$$, we can decompose $$$V$$$ into $$$V = V_1 \cup V_2 \cup ... \cup V_n$$$, where $$$V_i (1 \leq i \leq n)$$$ are pairwise disjoint non-empty sets. $$$V_1$$$ is a singleton which contains only one element: The centroid. We want to calculate some function $$$f(T)$$$, and we assume that the base case, where $$$V(T) = 1$$$, is easy to calculate. This assumption is not strict, because if we can't even deal with the case where $$$V(T)=1$$$, we can actually achieve nothing. The another important function is the merge function: $$$merge(V_1, V_2, ..., V_n)$$$.

Теги centroid decomposition

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en80 Английский Aveiro_quanyue 2023-03-30 12:21:13 2 Tiny change: 'the size if finite. W' -> 'the size is finite. W'
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en78 Английский Aveiro_quanyue 2023-03-29 05:17:51 8 Tiny change: 'ssions/40126314). I paste' -> 'ssions/40135200). I paste'
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en76 Английский Aveiro_quanyue 2023-03-28 18:38:23 9 Tiny change: 'ize of $v$'s parent) $\leq \f' -> 'ize of $v$) $\leq \f'
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en74 Английский Aveiro_quanyue 2023-03-28 17:57:37 1 Tiny change: 'rge)$ times. Therefor' -> 'rge)$ time. Therefor'
en73 Английский Aveiro_quanyue 2023-03-28 17:25:18 2 Tiny change: 'diameter mast pass th' -> 'diameter must pass th'
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en71 Английский Aveiro_quanyue 2023-03-28 17:11:42 0 (published)
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en1 Английский Aveiro_quanyue 2023-03-28 13:51:56 258 Initial revision (saved to drafts)