求阶乘
5! = 5 * 4! = 5 * 4 * 3! = …

public class Test{
    public static void main(String[] args) {
        int a = 6;
        System.out.println(fac(a));
    }

    public static int fac(int num){
        if (num <= 1){
            return 1;
        }else{
            return num * fac(num - 1);
        }
    }
}