剑指offer-把二叉树打印成多行

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。
输入：{8,6,10,5,7,9,11}
输出：[[8],[6,10],[5,7,9,11]]

思路一： 定义队列储存current当前节点，next_layer储存当前节点的子节点，current临时存储每一行的数据，并将下一行数据存储在next_layer，然后将结果append到result里，再输进新的数据。

class Solution:
    # 返回二维列表[[1,2],[4,5]]
    def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        current = [pRoot]
        next_layer = []
        result = []
        while current:
            for i in current:
                if i.left:
                    next_layer.append(i.left)
                if i.right:
                    next_layer.append(i.right)
            result.append([i.val for i in current])
            current,next_layer = next_layer,[]
        return result

思路二： 思路一的更精简的写法

class Solution:
    # 返回二维列表[[1,2],[4,5]]
    def Print(self, pRoot):
        # write code here
        if not pRoot:
            return []
        q = [pRoot]
        res = []
        while q:
            res.append([elem.val for elem in q])
            q=[leaf for node in q for leaf in (node.left,node.right) if leaf]
        return res