Maximum Depth of Binary Tree

The depth (height) of a binary tree is the number of nodes on the longest path from the root down to a leaf. Find it.

Tree format. A binary tree is given on one line in level order (top to bottom, left to right), with null for a missing child — the same way most interview platforms show trees. Trailing nulls are left out, and an empty tree is the single word null. Example: 3 9 20 null null 15 7 is

    3
   / \
  9   20
     /  \
    15   7

Input: one tree.

Output: the maximum depth (0 for an empty tree).

Constraints: 0 ≤ nodes ≤ 10⁴. The tree has at most 10⁴ nodes and its height is at most 500, so recursion is safe in every language.

Think about it: depth(node) = 1 + max(depth(left), depth(right)). Or count levels with a BFS queue.