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 }