https://leetcode.com/problems/maximum-depth-of-binary-tree/description/
Given the root of a binary tree, return its maximum depth.
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 1:
Input: root = [3,9,20,null,null,15,7]
Output: 3
example 2:
Input: root = [1,null,2]
Output: 2
The number of nodes in the tree is in the range [0, 10^4].
-100 <= Node.val <= 100