Choose and divide UVA - 10375——埃式筛法+组合数阶乘运算

/*
Eratosthenes筛法+组合数阶乘运算
*/
#include <cstdio>
#include <cstring>
#include <cmath>

using namespace std;

int tp, primes[10004], a[10004], e[10004];

void add_factorial(int n, int d);///表示把结果乘以(n!)^d,d = 1表示乘，d = -1表示除
void add_integer(int n, int d);

int main()
{
int p, q, r, s, i, j;
double ans;
///Eratosthenes筛法得到10000以内的所有素数
memset(a, 0, sizeof(a));
tp = 0, a[1] = 1, a[2] = 0;
for(i = 2; i <= 10000; i++){
if(!a[i]){
primes[tp++] = i;
for(j = i*2; j <= 10000; j += i){
a[j] = 1;
}
}
}
while(scanf("%d %d %d %d", &p, &q, &r, &s) != EOF){
memset(e, 0, sizeof(e));
add_factorial(p, 1);
add_factorial(q, -1);
add_factorial(p-q, -1);
add_factorial(r, -1);
add_factorial(s, 1);
add_factorial(r-s, 1);

ans = 1.0;
for(i = 0; i < tp; i++){
ans *= pow(primes[i], e[i]);
}
printf("%.5lf\n", ans);
}
return 0;
}
void add_factorial(int n, int d){
for(int i = 1; i <= n; i++)
add_integer(i, d);
}
void add_integer(int n, int d){
for(int i = 0; i < tp; i++){
while(n%primes[i] == 0){
n /= primes[i];
e[i] += d;
}
if(n == 1) break;
}
}