UVa 10375 (唯一分解定理) Choose and divide

#include <cstdio>
#include <cmath>
#include <cstring>

const int maxn = 10000;
int pri[maxn], cnt, e[maxn];    //e记录每个素数的质数
bool vis[maxn + 10];

void add_interger(int n, int d)    //乘以n的d次幂
{
for(int i = 0; i < cnt; ++i)
{
while(n % pri[i] == 0)
{
n /= pri[i];
e[i] += d;
}
if(n == 1) return;
}
}

void add_factorial(int n, int d)    //乘以(n!)的d次幂
{
for(int i = 2; i <= n; ++i)
add_interger(i, d);
}

int main()
{
//freopen("10375in.txt", "r", stdin);

int m = sqrt(maxn + 0.5);
for(int i = 2; i <= m; ++i) if(!vis[i])
for(int j = i*i; j <= maxn; j += i) vis[j] = true;
cnt = 0;
for(int i = 2; i < maxn; ++i) if(!vis[i]) pri[cnt++] = i;
//for(int i = 0; i < 10; ++i) printf("%d\n", pri[i]);

int p, q, r, s;
while(scanf("%d%d%d%d", &p, &q, &r, &s) == 4)
{
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);

double ans = 1.0;
for(int i = 0; i < cnt; ++i)
ans *= pow(pri[i], e[i]);

printf("%.5f\n", ans);
}

return 0;
}