LeetCode 834 Sum of Distances in Tree - Walkthrough

Compute sum of distances from each node to all others. Naive: $O(n^2)$. Rerooting: $O(n)$. Pass $1$ (post-order): compute $subtreeSize[v]$ and $subtreeSum[v]$ = sum of distances within subtree rooted at $v$. Pass $2$ (pre-order): for each node, combine subtree answer with "rest of tree" answer from parent.

When moving root from $u$ to child $v$: nodes in $v$'s subtree get 1 closer (there are $size[v]$), nodes outside get 1 farther (there are $n - size[v]$). New answer = old $\pm$ difference.