---
title: '104. Maximum Depth of Binary Tree'
description: Given the root of a binary tree, return its maximum depth
sidebar:
  label: 'Maximum Depth of Binary Tree'
  badge: 'Easy'
---

<Badge variant="accent">Tree DFS</Badge>

### 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`

## Approach

```mermaid
flowchart TD
  S(["maxDepth(root)"]) --> B{"root is None?"}
  B -- yes --> Z(["return 0"])
  B -- no --> L["l = maxDepth(root.left), r = maxDepth(root.right)"]
  L --> E(["return 1 + max(l, r) — this node adds one level above its deeper subtree"])
```

## Solution

```py
# 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:
        if not root:
            return 0

        return 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))
```

## Explanation

<YouTube id="hTM3phVI6YQ" title="Maximum Depth of Binary Tree - 3 Solutions - Leetcode 104 - Python" />