new_contents: # -*-coding:utf-8-*-
  
  """
  Question:
      Invert Binary Tree
  
      Invert a binary tree.
  
           4
         /   \
        2     7
       / \   / \
      1   3 6   9
      to
           4
         /   \
        7     2
       / \   / \
      9   6 3   1
  
      Trivia:
      This problem was inspired by this original tweet by Max Howell:
      Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so fuck off.
  
  Performance:
      1. Total Accepted: 40077 Total Submissions: 101621 Difficulty: Easy
      2. Your runtime beats 48.69% of python submissions.
  """
  
  
  # 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
          """
          if root is None:
              return
          root.left, root.right = root.right, root.left
          if root.left:
              self.invertTree(root.left)
          if root.right:
              self.invertTree(root.right)
          return root
  
  
  n1 = TreeNode(1)
  n2 = TreeNode(2)
  n3 = TreeNode(3)
  n4 = TreeNode(4)
  n6 = TreeNode(6)
  n7 = TreeNode(7)
  n9 = TreeNode(9)
  n4.left = n2
  n4.right = n7
  n2.left = n1
  n2.right = n3
  n7.left = n6
  n7.right = n9
  
  Solution().invertTree(n4)
  assert n4.left == n7
  assert n4.right == n2
  assert n7.left == n9
  assert n7.right == n6
  assert n2.left == n3
  assert n2.right == n1
  
subject: Add 226-invert-binary-tree.py