new_contents: 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 1 + max(self.maxDepth(root.left), self.maxDepth(root.right))
  
subject: 完成了104题:104. Maximum Depth of Binary Tree