python剑指offer系列把二叉树打印成多行

题目：
从上到下按层打印二叉树，同一层结点从左至右输出。每一层输出一行。

思路：

按层遍历，遍历完每层后更新

solution：# -*- coding:utf-8 -*-
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None

class Solution:
def Print(self, pRoot):
if not pRoot:
return []
nodeStack = [pRoot]
result = []
while nodeStack:
res = []
nextStack = []
for i in nodeStack:
res.append(i.val)
if i.left:
nextStack.append(i.left)
if i.right:
nextStack.append(i.right)
nodeStack = nextStack
result.append(res)
return result

# write code here