2020-04-07 Day 45 Leetcode: 104.

# 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:
        if root is None:
            return 0
        return self.helper(root)
    
    def helper(self,root):
        if root.left is None and root.right is None:
            return 1
        if root.left is None and root.right is not None:
            return self.helper(root.right)+1
        if root.left is not None and root.right is None:
            return self.helper(root.left)+1
        max_depth=max(self.helper(root.left), self.helper(root.right))+1
        return max_depth