py3 把二叉树打印成多行

思路:用两个列表分别保存当前层节点和下一层节点，结果.append([i.val for i in 当前层节点]),然后 当前层列表,下一层列表 = 下一层列表, [] 即可，直到当前层为空

class Solution:
    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