[LeetCode]题解（python）：103 Binary Tree Zigzag Level Order Traversal

题目来源

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https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).

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题意分析

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Input:一个二叉树

Output:一个list

Conditions:每层组成一个list，输出每层的集合，注意是zigzag的，意思是，当为偶数层时，尾部添加；当为奇数层时，首部添加。

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题目思路

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判断是不是偶数层，决定是否尾部添加

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AC代码（Python）

 # 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 preorder(self, root, level, res):
         if root:
             if len(res) < level + 1: res.append([])
             if level % 2 == 0:
                 res[level].append(root.val)
             else:
                 res[level].insert(0,root.val)
             self.preorder(root.left, level + 1, res)
             self.preorder(root.right, level + 1, res)
     def zigzagLevelOrder(self, root):
         """
         :type root: TreeNode
         :rtype: List[List[int]]
         """
         res = []
         self.preorder(root, 0, res)
         print res
         return res