32 - III. 从上到下打印二叉树 III

题目链接：https://leetcode-cn.com/problems/cong-shang-dao-xia-da-yin-er-cha-shu-iii-lcof/
难度：中等

描述：
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。

题解

# 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]]:
        ret = []
        q = deque()
        if root is not None:
            q.append(root)
        level = 1
        while q:
            size = len(q)
            temp_node = []
            while size > 0:
                cur = q.popleft()
                temp_node.append(cur.val)
                if cur.left:
                    q.append(cur.left)
                if cur.right:
                    q.append(cur.right)
                size -= 1
            if level % 2 == 1:
                ret.append(temp_node)
            else:
                ret.append(temp_node[::-1])
            level += 1
        return ret