226. Invert Binary Tree

Invert a binary tree.

     4   /   \  2     7 / \   / \1   3 6   9

to

     4   /   \  7     2 / \   / \9   6 3   1

# Definition for a binary tree node.# class TreeNode(object):#     def __init__(self, x):#         self.val = x#         self.left = None#         self.right = Noneclass Solution(object):    def invertTree(self, root):        """        :type root: TreeNode        :rtype: TreeNode        """        def change(r):            if r==None:                return None            else:                change(r.left)                change(r.right)                a=r.left                r.left=r.right                r.right=a                return r        return change(root)