队列

# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
# 
# @param root TreeNode类 
# @return int整型一维数组
#
class Solution:
    def PrintFromTopToBottom(self , root: TreeNode) -> List[int]:
        if not root: return
        queue,res = [root],[]
        while len(queue) > 0:
            # 弹出队头
            node = queue[0]
            queue = queue[1:]
            # 存储结果
            res.append(node.val)
            # 加入队列
            if node.left:  queue.append(node.left)
            if node.right:  queue.append(node.right)
        return res