LeetCode#226 Invert Binary Tree

Problem Definition:　　

　　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.

 1 class Solution:
 2     # @param {TreeNode} root
 3     # @return {TreeNode}
 4     def invertTree(self, root):
 5         if root==None:
 6             return root
 7         root.left,root.right=root.right,root.left
 8         self.invertTree(root.left)
 9         self.invertTree(root.right)
10         return root