num = input() num_list = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9] for i in range(len(num) - 1, -1, -1): if len(num_list) == 0: break if int(num[i]) in num_list: print(num[i], end='') num_list.remove(int(num[i])) 避免多余的循环