C语言：用1,3,5,7,能组成多少个互不相同且无重复数字的三位数?都是多少?

3位数?共24个整数,分别是:135.137.153.157.173.175.315.317.351.357.371.375.513.517.537.531.571.573.713.715.735.731.751.753 如果是4位数,也是24个,分别是:1357.1537.1573.1375.1753.1735.3157.3175.3517.3571.3751.3715.5137.5173.5317.5371.5713.5731.7135.7153.7351.7315.7513.7531