text: # Definition for a binary tree node.
  # 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 not root:
  			return 0
  		return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1