递归

public class MethodDemo09 {
    //2! 2*1
    //3! 3*2*1
    //5! 5*4*3*2*1
    public static void main(String[] args) {
        MethodDemo09 demo09 = new MethodDemo09();
        int result = demo09.recursion(5);
        System.out.println(result);
    }

    public int recursion(int n) {
        if ( n == 1 ) {
            return 1;
        } else {
            return n * recursion(n-1);
        }
    }
}

用for循环求阶乘

public class MethodDemo10 {
    public static void main(String[] args) {
        MethodDemo10 demo10 = new MethodDemo10();
        demo10.recursion();
    }

    //用for循环求阶乘
    public void recursion() {
        Scanner sc = new Scanner(System.in);
        System.out.println("请输入一个整数：");
        int num = sc.nextInt();
        if ( num == 1 ) {
            System.out.println(num + "的阶乘为：1");
            return;
        }
        int result = 1;
        for ( int i = 2; i <= num; i++ ) {
            result = result * i;
        }
        System.out.println(num + "的阶乘为：" + result);
    }
}

当输入的数为 1 时：

当输入的数为 2 时：