Maximum Depth of Binary Tree

Define depth(node) as the number of nodes on the longest path from node down to a leaf. The base case is depth(null) = 0. For a real node, return 1 + max(depth(node.left), depth(node.right)). This directly mirrors the tree's recursive structure and visits each node once, so time is O(n). Recursive stack space is O(h), where h is the tree height; a skewed tree can therefore use O(n) stack space. An iterative BFS alternative processes the tree level by level and increments a counter after each level. BFS also takes O(n) time but may hold up to the tree's maximum width in memory. Clarify whether depth counts nodes or edges; this problem's conventional answer counts nodes, giving depth one for a single-node tree and zero for an empty tree.