【欧拉函数】BZOJ2818-GCD

怎么漏了这一道……本来想要水一水，结果忘记了φ[1]=1，果然要滚一遍前面的知识……

 #include<iostream>
 #include<cstdio>
 #include<cstring>
 #include<algorithm>
 using namespace std;
 const int MAXN=+;
 typedef long long ll;
 int n;
 int prime[MAXN],pnum=;
 int phi[MAXN];
 ll sum[MAXN];

 void get_phi(int maxn)
 {
     memset(phi,,sizeof(phi));
     memset(sum,,sizeof(sum));
     phi[]=sum[]=;
     for (int i=;i<=maxn;i++)
     {
         if (phi[i]==)
         {
             prime[++pnum]=i;
             phi[i]=i-;
         }
         for (int j=;j<=pnum;j++)
         {
             if (i*prime[j]>maxn) break;
             if (i%prime[j]==) phi[i*prime[j]]=phi[i]*prime[j];
                 else phi[i*prime[j]]=phi[i]*(prime[j]-);
         }
         sum[i]=sum[i-]+phi[i];
     }
 }

 int main()
 {
     scanf("%d",&n);
     get_phi(n);
     ll ans=;
     for (int i=;i<=pnum;i++)
     {
         ans+=sum[n/prime[i]]*-;
     }
     printf("%lld",ans);
     return ;
 }