new_contents: # 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 root is None:
              return 0
          else:
              return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
  
subject: add solution-104-maximum-depth-of-binary-tree.