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 invertTree(self, root):
          """
          :type root: TreeNode
          :rtype: TreeNode
          """
          self.dfs(root)
          
      def dfs(self, node):
          left = node.left
          right = node.right
          node.left = right
          node.right = left
          if (left!=None): self.dfs(left)
          if (right!=None): self.dfs(right)
subject: add file