欧拉函数之和（51nod 1239）

#include<cstdio>
#include<iostream>
#define N 2000010
#define ha 2333333
#define mod 1000000007
#define ni 500000004
#define lon unsigned long long
using namespace std;
int phi[N],prime[N],cnt,tot,head[N],vis[N];
lon n,sum[N];
struct node{int pre;lon x,v;}e[N];
void add(int u,lon v,lon x){
    e[++cnt].v=v;e[cnt].x=x;e[cnt].pre=head[u];head[u]=cnt;
}
void get_prime(){
    phi[1]=1;
    for(int i=2;i<N;i++){
        if(!vis[i]) vis[i]=1,prime[++tot]=i,phi[i]=i-1;
        for(int j=1;j<=tot&&i*prime[j]<N;j++){
            vis[i*prime[j]]=1;
            phi[i*prime[j]]=phi[i]*(prime[j]-1);
            if(i%prime[j]==0){
                phi[i*prime[j]]=phi[i]*prime[j];
                break;
            }
        }
    }
    for(int i=1;i<N;i++) sum[i]=(sum[i-1]+phi[i])%mod;
}
lon solve(lon x){
    if(x<N) return sum[x];
    lon ans=0,k=x%ha,last;
    for(int i=head[k];i;i=e[i].pre)
        if(e[i].v==x) return e[i].x;
    for(lon i=2;i<=x;i=last+1){
        last=x/(x/i);
        ans=(ans+(last-i+1)%mod*solve(x/i)%mod)%mod;
    }
    ans=((x%mod*(x+1)%mod)%mod*ni%mod-ans+mod)%mod;
    add(k,x,ans);
    return ans;
}
int main(){
    get_prime();
    cin>>n;
    cout<<solve(n);
    return 0;
}