c语言学子中的几个比较经典的程序，包括有1、2、3、4个数字，能组成多少个互不相同且无重复数字的三位数？都是多少？等 -c language students a few more classic procedures, including 1,2,3,4 figures, can be formed with each other with no repeating three-digit numbers? How many are? Wait