Maximum Depth of Binary Tree

Given the root of a binary tree, return its maximum depth.

LeetCode: Maximum Depth of Binary Tree

A binary tree's maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

Example:

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

    3
   / \
  9  20
    /  \
   15   7