剑指 Offer 32 - III. 从上到下打印二叉树 III

请实现一个函数按照之字形顺序打印二叉树，即第一行按照从左到右的顺序打印，第二层按照从右到左的顺序打印，第三行再按照从左到右的顺序打印，其他行以此类推。

例如:
给定二叉树: [3,9,20,null,null,15,7],

3<br />   / \<br />  9  20<br />    /  \<br />   15   7<br />返回其层次遍历结果：

[
[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 levelOrder(self, root: TreeNode) -> List[List[int]]:
        if root is None:
            return []
        queue = []
        queue.append(root)
        res = []
        path = []
        temp = []
        flag = 1
        while queue:
            path = []
            temp = []
            while queue != []:
                temp.append(queue.pop(0))
            while temp != []:
                root = temp.pop(0)
                path.append(root.val)
                if root.left is not None:
                    queue.append(root.left)
                if root.right is not None:
                    queue.append(root.right)
            if flag % 2 == 1:
                res.append(path[:])
            else:
                res.append(path[::-1])
            flag += 1
        return res