LeetCode 834 Sum of Distances in Tree - Two Passes

Pass $1$ (down): Root at node $0$. Compute $\text{count}[v]$ = nodes in $v$'s subtree, and $\text{sum}[v]$ = sum of distances from $v$ to all nodes in its subtree. $\text{count}[v] = 1 + \sum \text{count}[\text{child}]$ and $\text{sum}[v] = \sum (\text{sum}[\text{child}] + \text{count}[\text{child}])$.

Pass $2$ (up): Starting from root, propagate answers to children using the formula from the previous unit.

The first pass collects information from leaves to root. The second pass distributes answers from root to leaves.