进行一个子集容斥，钦定叶子集合，展开计算式： $$ \sum_{S,T\subseteq U, S\cap T = \varnothing} f(S) g(T) (-2)^{|S\cap T|} $$
转换计算顺序（按编号考虑各点属于 $S$ 还是 $T$ 还是两者皆否），使用 DP 计算和式即可。
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Discussion #837 for Problem #4007. 树
Type: Editorial
Status: Open
Posted by: alpha1022
Posted at: 2026-01-28 02:13:53
Last updated: 2026-01-28 02:14:00
简要题解
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