103.Binary Tree Zigzag Level Order Traversal（层序遍历）

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).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its zigzag level order traversal as:

[
[3],
[20,9],
[15,7]]

# Definition for a binary tree node.
class TreeNode:
    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None

class Solution:
    def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        level = []
        if root is None:
            return []
        level.append(root)
        while len(level):
            temp = [] #为了保存每一层的结果
            l = len(level)
            for i in range(l): #为了遍历每一层的节点
                node = level.pop(0)
                temp.append(node.val)
                if node.left:
                    level.append(node.left)
                if node.right:
                    level.append(node.right)
            res.append(temp)
        for i in range(len(res)):
            if i%2==1:
                res[i].reverse()
        return res