public class Solution
extends Object
104 - Maximum Depth of Binary Tree\.
Easy
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
**Example 3:**
**Input:** root = []
**Output:** 0
**Example 4:**
**Input:** root = [0]
**Output:** 1
**Constraints:**
* The number of nodes in the tree is in the range [0, 104].
* `-100 <= Node.val <= 100`