阶乘和

#include <stdio.h>

// __int64的范围是 [0, 2^64),即0~18446744073709551615(约1800亿亿)
static unsigned __int64 sum_fac(int n);

int main(void)
{
  printf("test sum_fac function.\n\n");
  for(unsigned int k=0; k<20; k++)
  {
    printf("k=%dth e sum is: %llu\n",k,sum_fac(k)); 
  }
  
  getchar();
  return 0;
}

/* long sum_fac(int n)
作用：求解1!+2!+3!+...+N! ，即阶乘和。
*/

unsigned __int64 sum_fac(int n)
{
  if(0 == n)
    return 1L;

  unsigned __int64 sum = 0;

  //Calc
  unsigned __int64 tmp = 1;
  for(unsigned int i=1;i<=n;i++)
  {
    tmp *= i;
    sum += tmp;
  }

  return sum;
}