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题目：
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.
链接： http://leetcode.com/problems/invert-binary-tree/
一刷
1 class Solution(object): 2 def invertTree(self, root): 3 if not root: 4 return root 5 current = [root] 6 next_level = [] 7 while current: 8 for idx, node in enumerate(current): 9 if node.left: 10 next_level.append(node.left) 11 if node.right: 12 next_level.append(node.right) 13 node.left, node.right = node.right, node.left 14 current, next_level = next_level, [] 15 return root
一开始最主要的一步交换左右忘记了，而且入next时候其实左子树和右子树的顺序无所谓。曾经想把第3，4行去掉加入循环体内，但是需要额外判断是否为空，没有必要为了唯一一种特殊情况加入额外的步骤。
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原文地址：http://www.cnblogs.com/panini/p/5642234.html
