https://www.nowcoder.com/practice/7fe2212963db4790b57431d9ed259701?tpId=13&tqId=11175&tPage=2&rp=2&ru=%2Fta%2Fcoding-interviews&qru=%2Fta%2Fcoding-interviews%2Fquestion-ranking

題目描述
從上往下打印出二叉樹的每個節點，同層節點從左至右列印。

層序遍歷：

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
 

#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    # 返回從上到下每個節點值列表，例：[1,2,3]
    def PrintFromTopToBottom(self, root):
        # write code here
        if not root:
            return []
        queue, res = [root], []
        while queue:
            for
 
 i in range(len(queue)):
                node = queue.pop(0)
                if node.left:
                    queue.append(node.left)
                if node.right:
                    queue.append(node.right)
                res.append(node.val)
        return res