# Maximum Depth of Binary Tree

Given a binary tree, find its maximum depth.

The maximum depth is the number of nodes along the longest path from the root node down to the farthest leaf node.

## Example

Given a binary tree as follow:

```
  1
 / \ 
2   3
   / \
  4   5
```

The maximum depth is 3.

## Solution

分治的精髓就是，把根节点，左子树，右子树的关系进行分析。

```
"""
Definition of TreeNode:
class TreeNode:
    def __init__(self, val):
        self.val = val
        self.left, self.right = None, None
"""
class Solution:
    """
    @param root: The root of binary tree.
    @return: An integer
    """ 
    def maxDepth(self, root):
        # write your code here
        if root is None:
            return 0
        left = self.maxDepth(root.left)
        right = self.maxDepth(root.right)

        return max(left,right) + 1
```