Leetcode练习(Python)：栈类：第103题：二叉树的锯齿形层次遍历：给定一个二叉树，返回其节点值的锯齿形层次遍历。（即先从左往右，再从右往左进行下一层遍历，以此类推，层与层之间交替进行）。

# 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: TreeNode) -> List[List[int]]:
        if not root:
            return []
        result = []
        current_level = [root]
        index = 0
        while current_level:
            auxiliary = []
            next_level = []
            for node in current_level:
                auxiliary.append(node.val)
                if node.left:
                    next_level.append(node.left)
                if node.right:
                    next_level.append(node.right)
            if index % 2 == 1:
                result.append(auxiliary[::-1])
            if index % 2 == 0:
                result.append(auxiliary)
            index += 1
            current_level = next_level
        return result