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

题目来源

---

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).

---

题意分析

---

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。

---

题目思路

---

判断是不是偶数层，决定是否尾部添加

---

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