参考代码：

class Solution:
    def previousPermution(self, num):
        for i in range(len(num)-2, -1, -1):
            if num[i] < num[i+1]:
                break
        else:
            num.reverse()
            return num
        for j in range(len(num)-1, i, -1):
            if num[j] > num[i]:
                num[i], num[j] = num[j], num[i]
                break
        for j in range(0, (len(num) - i)//2):
            num[i + j + 1], num[len(num)-j-1] = num[len(num)-j-1], num[i+j+1]
        return num