Maximum Depth of Binary Tree

Given the root of a binary tree, return the maximum depth, which is the number of nodes on the longest path from the root to any leaf.

Examples
Input: [3,9,20,null,null,15,7]
Output: 3
Hints
Related Problems

Maximum Depth of Binary Tree

Given the root of a binary tree, return the maximum depth, which is the number of nodes on the longest path from the root to any leaf.