```
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution(object):
def maxDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if root is None:
return 0
def calc_depth(tree):
if tree.left is None and tree.right is None:
return 1
elif tree.left is None:
return calc_depth(tree.right) + 1
elif tree.right is None:
return calc_depth(tree.left) + 1
else:
return max([calc_depth(tree.left), calc_depth(tree.right)]) + 1
return calc_depth(root)```
```