c - 计算1到20的阶乘

 1 #include <stdio.h>

 2 

 3 /*

 4 题目：求 1+2!+3!+...+20!的和

 5 */

 6 unsigned long long int

 7 factorial(long n) {

 8     unsigned long long int tmp = 1;    //每一个数的阶乘,如,1!,2!,3!...20!.

 9     unsigned long long int sum = 0;    //总和,即1+2!+3!+...+20!的和.

10     for(long long int i = 1; i <= n; i++)     {

11         tmp *= i;

12         sum += tmp;

13     }

14 

15     return sum;

16 }

17 

18 int

19 main(void) {

20     unsigned long long int r = factorial(20);

21     printf("%llu\n", r);        

22 

23 /*

24 _int64 fact = 1,i,sum = 0;

25     for(i = 1; i < 21; ++i)    {

26         fact *= i;

27         sum += fact;

28     }

29     printf("sum = %I64d\n",sum);

30 */

31 }