LeetCode 104 Maximum Depth of Binary Tree

# 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):
        """
        :type root: TreeNode
        :rtype: int
        """
        if root is None:
            return 0
        ld = self.maxDepth(root.left)
        rd = self.maxDepth(root.right)
        return 1 + max(ld, rd)