BZOJ 4146 Divisors

#include<iostream>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define maxn 2000500
using namespace std;
int n,a[maxn],cnt[maxn],mx=0;
long long ret=0;
int main()
{
    scanf("%d",&n);
    memset(cnt,0,sizeof(cnt));
    for (int i=1;i<=n;i++)
    {
        scanf("%d",&a[i]);
        mx=max(mx,a[i]);
        cnt[a[i]]++;
    }
    sort(a+1,a+n+1);
    for (int i=1;i<=n;i++)
    {
        if (a[i]==a[i-1]) continue;
        long long regis=0;
        for (int j=2*a[i];j<=mx;j+=a[i])
            regis+=cnt[j];
        regis+=cnt[a[i]]-1;
        ret+=regis*cnt[a[i]];
    }
    printf("%lld\n",ret);
    return 0;
}