CF839D Winter is here

原题

#include<bits/stdc++.h> 
#define r register 
using namespace std;
const int N=1e6+5,Mod=1e9+7;
int n,fac[N],cnt[N],dp[N],cur,ans;
int main() {
    scanf("%d",&n);
    fac[0]=1;
    for (r int i=1;i<=n;i++) fac[i]=2ll*fac[i-1]%Mod;
    for (r int i=1,x;i<=n;i++)
        scanf("%d",&x),cnt[x]++;
    for (r int i=N-1;i>1;i--) {
        cur=0;
        for (r int j=i;j<N;j+=i) cur+=cnt[j];
        if (!cur) continue;
        dp[i]=1ll*fac[cur-1]*cur%Mod;
        for (r int j=i+i;j<N;j+=i) 
            dp[i]=(dp[i]-dp[j]+Mod) % Mod;
        ans=(ans+1ll*dp[i]*i%Mod)%Mod;
    }
    printf("%d\n",ans);
    return 0;
}