from collections import deque
class Solution(object):
    def printFromTopToBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        res = []
        reverse = False # 初始开关
        queue = deque([root])
        while queue:
            level = []
            for _ in range(len(queue)):
                node = queue.popleft()
                level.append(node.val)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
            if reverse: # 第一层不用反转
                level.reverse()
            reverse = not reverse 
            res.append(level)
        return res