「luogu2714」四元组统计

#include<bits/stdc++.h>
#define ll long long
using namespace std;
const int N=10010,maxn=10000;
int n,a[N],u[N],p[N],totp;
ll f[N];
bool isp[N];
inline ll calc(ll k){return k*(k-1)*(k-2)*(k-3)/4/3/2;}
void solve(){
    memset(a,0,sizeof(a));
    memset(f,0,sizeof(f));
    int t;
    ll ans=0;
    for(int i=1;i<=n;i++) scanf("%d",&t),a[t]++;
    for(int i=1;i<=maxn;i++){
        for(int j=i;j<=maxn;j+=i) f[i]+=a[j];
        if(f[i]>3) f[i]=calc(f[i]);
        else f[i]=0;
    }
    for(int i=1;i<=maxn;i++) ans+=f[i]*u[i];
    printf("%lld
",ans);
    return;
}
int main(){
    isp[1]=1,u[1]=1;
    for(int i=2;i<=maxn;i++){
        if(!isp[i]) p[++totp]=i,u[i]=-1;
        for(int j=1;j<=totp&&p[j]*i<=maxn;j++){
            isp[p[j]*i]=1;
            if(!(i%p[j])) break;
            u[p[j]*i]=-u[i];
        }
    }
    while(scanf("%d",&n)!=EOF) solve();
    return 0;
}