Maximum Depth of Binary Tree

Problem

Return the maximum depth of the binary tree.

Example 1

Input: root = [3,9,20,null,null,15,7]
Output: 3

Constraints

• The number of nodes is in the range [0, 10^4]

Notes

• Focus on the underlying binary tree pattern.
• Write the cleanest correct solution before optimizing further.
• Hidden tests cover larger edge cases than the visible examples.