# 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
```

 

**Constraints:**

* The number of nodes in the tree is in the range `[0, 104]`.
* `-100 <= Node.val <= 100`

```python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, val=0, left=None, right=None):
#         self.val = val
#         self.left = left
#         self.right = right
class Solution:
    def maxDepth(self, root: Optional[TreeNode]) -> int:
        result = 0
        def recursion(root, depth):
            nonlocal result
            if root is None:
                if depth > result:
                    result = depth
                return
            recursion(root.left, depth+1)
            recursion(root.right, depth+1)
        recursion(root, 0)
        return result
```