## Description #

Difficulty: **简单**

Related Topics: Tree, Depth-First Search, Breadth-First Search, Binary Tree

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
```

**Constraints:**

- The number of nodes in the tree is in the range [0, 10
^{4}]. `-100 <= Node.val <= 100`

## Solution #

Language: **Go**

IOI