本文章由 WyOJ Shojo 从洛谷专栏拉取，原发布时间为 2024-02-15 14:55:29

设三个点为 $a, b, c$，$x =\text{lca}(a, b), y = \text{lca}(b, c), z = \text{lca}(a, c)$，不妨设 $x = y$，那么总距离为：

$$ d(a) - d(x) + d(b) + d(c) - 2 \times d(z) + d(z) - d(x) $$

整理得

$$d(a) + d(b) + d(c) - 2\times d(x) - d(z)$$

也即 $$d(a) + d(b) + d(c) - d(x) - d(y) - d(z)$$