题目：求1+2!+3!+...+20!的和
public class Twenty_firstFactorialSum {
static long sum = 0;
static long fac = 0;
public static void main(String[] args) {
   long sum = 0;
   long fac = 1;
   for(int i=1; i<=10; i++) {
    fac = fac * i;
    sum += fac;
   }
   System.out.println(sum);
}
}

题目：求1+2!+3!+...+20!的和
public class Twenty_firstFactorialSum {
static long sum = 0;
static long fac = 0;
public static void main(String[] args) {
   long sum = 0;
   long fac = 1;
   for(int i=1; i<=10; i++) {
    fac = fac * i;
    sum += fac;
   }
   System.out.println(sum);
}
}