Python：把二叉树打印成多行

牛客网上的剑指 offer的在线编程：

题目描述

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。

# -*- 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 not pRoot:
            return []
        res, tree = [], [pRoot]
        while tree:
            row, sub_tree = [], []
            for item in tree:
                row.append(item.val)
                if item.left:
                    sub_tree.append(item.left)
                if item.right:
                    sub_tree.append(item.right)
            res.append(row)
            tree = sub_tree
        return res