Maximum Depth of Binary Tree

Given a binary tree, return its maximum depth (the number of nodes along the longest path from root to a leaf).

Example

Input (level-order, null for missing): 3 9 20 null null 15 7 Output: 3

Approach

The depth of a tree is 1 (for the root itself) plus the deeper of its two subtrees' depths — a two-line recursive definition. An empty tree has depth 0.

Input format

Line 1: the tree in level-order, space-separated, null for a missing child (this is LeetCode's own tree-input convention)

Print the maximum depth.