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 = 1;    //每一个数的阶乘,如,1!,2!,3!...20!.
unsigned long long int sum = 0;    //总和,即1+2!+3!+...+20!的和.
for(long long int i = 1; i <= n; i++)     {
tmp *= i;
sum += tmp;
}

return sum;
}

int
main(void) {
unsigned long long int r = factorial(20);
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);
*/
}