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


class Solution:
    def levelOrder(self, root: TreeNode) -> List[List[int]]:
        if not root:
            return []
        queue = deque([root])
        res = []
        reverse = False
        while queue:
            size = len(queue)
            part = []
            while size > 0:
                size -= 1
                tree = queue.popleft()
                part.append(tree.val)
                children = [tree.left, tree.right]
                if any(children):
                    for child in children:
                        if child:
                            queue.append(child)
            if reverse:
                res.append(part[::-1])
            else:
                res.append(part)
            reverse = not reverse
        return res