阶乘。例子：1!+2!+3!+...+N!

    public static void main(String[] args) {
        //1!+2!+3!+...+N!
        System.out.println("阶乘结果之和"+test(5));
        System.out.println("阶乘结果之和"+sum(5));
    }
    //方法一
    public static int sum(int n) {
        int sum = 0;
        for (int i = 1; i <= n; i++) {
            sum += factorial(i);
        }
        return sum;
    }
    public static int factorial(int n) {
        int sum = 1;
        for (int i = 1; i <= n; i++) {
            sum *= i;
        }
        return sum;
    }
    //方法二
    public static int test(int num){
        int sum = 0;
        int temp = 1;
        for(int i = 1; i <= num; i++){
            temp = temp*i;
            sum += temp;
            System.out.print(sum+",");
        }
        return sum;
    }