Tree Diameter / Longest Path

LeetCode 543 / 1245. Longest path between any two nodes in a tree. DFS returns the deepest descendant chain; at each node, the diameter candidate is the sum of the two longest child chains. Requirements Input: binary tree (LC 543) or general N-ary tree (LC 1245). Output: number of edges on the longest path between any two nodes. Single DFS: at each node return 1 + max(child_depths); meanwhile update a global best with sum_of_top_two_child_depths. Examples LC 543: tree [1,2,3,4,5] → 3 (path 4→2→1→3 or 5→2→1→3). N-ary tree variant: same logic but track the top-two child chain depths per node. Notes Common bug: returning the diameter from the DFS instead of the depth — the recurrence needs depth, the answer is a global. N-ary variant: maintain best_first and best_second while iterating children to avoid an O(k log k) sort. A different problem class: "longest monotonic path in BST" (sorted values along the path) — has the same DFS shape but with directional invariants. Preparation Write LC 543 from memory in under 8 min. Drill the N-ary variant (LC 1245) — Meta's preferred follow-up. Be ready for the "longest monotonic path in BST" twist as a third variant.