我是f表示在i为根子树中建立j个伐木场，然后i下游最近的伐木场在i往下数第k个。

我不是用背包，而是对每个i,j,k对儿子们用了二分。后来发现超时，又加了个类似线段树的记忆化-_-|||结果就ac了……

成功地把这题AC率降低25个百分点

f表示以I为根结点，子树中建K个伐木场，在I上面最近的伐木场为K的最小费用，对于每个I，用背包解决。另外，如果不是像李永刚那么强的话，最好分是不是在I建伐木场两种情况讨论，这样方便一些

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这题难度也三啊？

树形Dp即可。

注意这里需用三维状态，否则会有后效性。

提示：

设立状态 f[from][to][k]表示以from为根的子树建立k个伐木场，且有一个祖先伐木场为to的最小费用。

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