I am a beginner of Programming and Python is my first language.

Just happy to solve this challenge myself so I want to share this simple and easy solution and my joy.

Thinking:

I have to count each level of node and find the max depth so that I figure out this simple formula:

f(n) = 1 + max(f(n.left),f(n.right))

1 is root as a default.

```
class Solution(object):
def maxDepth(self, root):
"""
:type root: TreeNode
:rtype: int
"""
if not root:
return 0
Sum = 1
if root.left == None and root.right == None:
return Sum
return Sum + max(self.maxDepth(root.left),self.maxDepth(root.right))
```