DP Method - How It Works

For each node $u$, run DFS to compute the depth of each child's subtree. The depth is $1$ plus the maximum depth of any child. Track the two deepest child subtrees.

The diameter through node $u$ is the sum of the two deepest paths going down. Compare this to the global maximum. After processing all nodes, you have the diameter.

This single DFS computes the diameter in $O(n)$ time. It is useful when you also need subtree information or when the problem combines diameter with other DP states.

Space complexity is $O(n)$ for the data structures used.