2019江西省赛C

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int maxn=1e5+10;
vector<int> a,b,g[maxn];
int num[maxn];
int main()
{
    int n;
    cin>>n;
    for(int i=0;i<n;i++){
        int x;
        cin>>x;
        num[x]++;
        a.push_back(x);
    }
    sort(a.begin(),a.end());
    a.erase(unique(a.begin(),a.end()),a.end());
    for(int i=0;i<=10000;i++){
        if(num[i]>=2)
            b.push_back(i);
    }
    for(int i=1;i<=10000;i++){
        for(int j=0;j<(int)b.size();j++){
            if(__gcd(i,b[j])==1)
                g[i].push_back(b[j]);
        }
    }
    ll ans=0;
    for(int i=0;i<(int)a.size();i++){
        for(int j=i+1;j<(int)a.size();j++){
            int limit = (a[j]-a[i])%2==0?(a[j]-a[i])/2+1:(a[j]-a[i]+1)/2;
            int d = __gcd(a[i],a[j]);
            //cout<<a[i]<<" "<<a[j]<<endl;
            if(d==1){
                int index = lower_bound(b.begin(),b.end(),limit)-b.begin();
                ans=ans+(int)b.size()-index;
                if(num[a[i]]==2&&a[i]>=limit)ans--;
                if(num[a[j]]==2&&a[j]>=limit)ans--;
            }
            else{
                int index = lower_bound(g[d].begin(),g[d].end(),limit)-g[d].begin();
                ans=ans+(int)g[d].size()-index;
            }
            //cout<<"now ans= "<<ans<<endl;
        }
    }
    cout<<ans<<endl;
}