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。
二叉树的层次遍历，对于每一层的元素放在同一个列表中即可
# -*- coding:utf-8 -*-
# class TreeNode:
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
class Solution:
# 返回二维列表[[1,2],[4,5]]
def Print(self, pRoot):
# write code here
if pRoot is None:
return []
p = [pRoot]
res = []
while p:
node = []
li = []
for x in p:
if x.left:
node.append(x.left)
if x.right:
node.append(x.right)
li.append(x.val)
p = node
res.append(li)
return res