题目描述
从上往下打印出二叉树的每个节点，同层节点从左至右打印。

# -*- coding:utf-8 -*-
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None
class Solution:
    # 返回从上到下每个节点值列表，例：[1,2,3]
    def PrintFromTopToBottom(self, root):
        # write code here
        if not root:
            return []
        result = []
        tmp = [root]
        while len(tmp):
            cur = tmp.pop(0)
            result.append(cur.val)
            if cur.left:
                tmp.append(cur.left)
            if cur.right:
                tmp.append(cur.right)
        return result