c - 计算1到20的阶乘

 #include <stdio.h>

 /*
 题目：求 1+2!+3!+...+20!的和
 */
 unsigned long long int
 factorial(long n) {
     unsigned long long int tmp = ;    //每一个数的阶乘,如,1!,2!,3!...20!.
     unsigned long long int sum = ;    //总和,即1+2!+3!+...+20!的和.
     for(long long int i = ; i <= n; i++)     {
         tmp *= i;
         sum += tmp;
     }

     return sum;
 }

 int
 main(void) {
     unsigned long long int r = factorial();
     printf("%llu\n", r);        

 /*
 _int64 fact = 1,i,sum = 0;
     for(i = 1; i < 21; ++i)    {
         fact *= i;
         sum += fact;
     }
     printf("sum = %I64d\n",sum);
 */
 }