19 - Hard - 螺旋矩阵

给定一个包含 m x n 个元素的矩阵（m 行, n 列），请按照顺时针螺旋顺序，返回矩阵中的所有元素。

示例 1:

输入:
[
[ 1, 2, 3 ],
[ 4, 5, 6 ],
[ 7, 8, 9 ]
]
输出: [1,2,3,6,9,8,7,4,5]
示例 2:

输入:
[
[1, 2, 3, 4],
[5, 6, 7, 8],
[9,10,11,12]
]
输出: [1,2,3,4,8,12,11,10,9,5,6,7]

class Solution:
    def spiralOrder(self, matrix):
        """
        :type matrix: List[List[int]]
        :rtype: List[int]
        """
        ret = []
        while matrix:
            ret += matrix.pop(0)
            if matrix and matrix[0]:
                for row in matrix:
                    ret.append(row.pop())
            if matrix:
                ret += matrix.pop()[::-1]
            if matrix and matrix[0]:
                for row in matrix[::-1]:
                    ret.append(row.pop(0))
        return ret