从上往下打印二叉树

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
        ret = [] 
        if root == None:
            return ret
        #广度优先搜索
        #bfs存储当前层的节点
        #tbfs存储当前层的左右子节点
        bfs = [root]
        while(bfs):
            tbfs = []
            for node in bfs:
                ret.append(node.val)
                if node.left:
                    tbfs.append(node.left)
                if node.right:
                    tbfs.append(node.right)
            bfs = tbfs[:]
        return ret