求阶乘和（二次递归）

public class test72 {
    /**
     * 求阶乘和
     * @param args
     */
    public static void main(String[] args) {
        int sum = fac2(4);
        System.out.println(sum);
    }

    public static Integer fac1(int n) {
        if(n == 1) {
            return 1;
        }
          return n * fac1(n - 1);
    }
    public static Integer fac2(int n) {
        int sum = 0;
        for (int i = 1; i <= n; i ++) {
            sum += fac1(i);
        }
        return sum;
    }
}