new_contents: # Definition for a binary tree node.
  # class TreeNode:
  #     def __init__(self, x):
  #         self.val = x
  #         self.left = None
  #         self.right = None
  
  
  class Solution:
      # @param {TreeNode} root
      # @return {TreeNode}
  
      def invertTree(self, root):
          if not root:
              return root
          root.left, root.right = (self.invertTree(root.right),
                                   self.invertTree(root.left))
          return root
  
subject: add solution for Invert Binary Tree