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.

class Solution:
    # @param {TreeNode} root
    # @return {TreeNode}
    def invertTree(self, root):
        if root==None:
            return root
        root.left,root.right=root.right,root.left
        self.invertTree(root.left)
        self.invertTree(root.right)
        return root