DFS, just easy to understand^_^

```
class Solution:
# @param {TreeNode} root
# @return {integer}
def maxDepth(self, root):
return self.dfs(root)
def dfs(self, node):
if not node:
return 0
elif (not node.left) and (not node.right):
return 1
else:
return 1 + max(self.dfs(node.left), self.dfs(node.right))
```