題目：

從上到下按層列印二叉樹，同一層結點從左至右輸出。每一層輸出一行。

思路：

按層遍歷，遍歷完每層後更新

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