104. Maximum Depth of Binary Tree

```python
# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
    def maxDepth(self, root: TreeNode) -> int:
        return 1 + max(map(self.maxDepth, (root.left, root.right))) if root else 0
```

**递归**