洛谷 P2714 四元组统计

错误记录：42行N写成n；事实上除了读入以外，此题的小n没有任何用途

#include<cstdio>
#include<algorithm>
#include<cstring>
#include<vector>
using namespace std;
#define fi first
#define se second
#define mp make_pair
#define pb push_back
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> pii;
const ll N=10000;
ll b[10100],ans;
ll mu[10100],prime[10100],len,n;
bool nprime[10100];
ll F(ll x)
{
    ll i,ans=0;
    for(i=x;i<=N;i+=x)    ans+=b[i];
    return __int128(ans)*(ans-1)*(ans-2)*(ans-3)/24;
}
int main()
{
    ll i,j,t;
    mu[1]=1;
    for(i=2;i<=N;i++)
    {
        if(!nprime[i])    prime[++len]=i,mu[i]=-1;
        for(j=1;j<=len&&i*prime[j]<=N;j++)
        {
            nprime[i*prime[j]]=1;
            if(i%prime[j]==0)    {mu[i*prime[j]]=0;break;}
            else    mu[i*prime[j]]=-mu[i];
        }
    }
    while(scanf("%lld",&n)==1)
    {
        memset(b,0,sizeof(b));
        for(i=1;i<=n;i++)    scanf("%lld",&t),b[t]++;
        ans=0;
        for(i=1;i<=N;i++)
        {
            ans+=mu[i]*F(i);
        }
        printf("%lld
",ans);
    }
    return 0;
}