剑指offer顺时针打印矩阵python

# -*- coding:utf-8 -*-
class Solution:
    # matrix类型为二维列表，需要返回列表
    def printMatrix(self, matrix):
        if not matrix:
            return 
        x0,y0 = 0,0
        xn = len(matrix) - 1
        yn = len(matrix[0]) - 1
        ans = []
        while x0 <=xn and y0 <=yn:
            for i in range(y0,yn+1):
                ans.append(matrix[x0][i])
            for i in range(x0+1,xn+1):
                ans.append(matrix[i][yn])
            if xn > x0: #不是单一行
                for i in range(yn-1,y0-1, -1):
                    ans.append(matrix[xn][i])
            if yn > y0:
                for i in range(xn-1,x0, -1):
                    ans.append(matrix[i][y0])
            x0 += 1
            y0 += 1
            xn -= 1
            yn -= 1
        return ans