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:**

![](https://assets.leetcode.com/uploads/2020/11/26/tmp-tree.jpg)

**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 <code>[0, 10<sup>4</sup>]</code>.
*   `-100 <= Node.val <= 100`