<LeetCode><Easy>226Invert a 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.

#Python2 40ms

# 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):
if not root:
return
if root.left:
self.invertTree(root.left)
if root.right:
self.invertTree(root.right)
root.left,root.right=root.right,root.left
return root