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……
1 #include<iostream> 2 #include<cstdio> 3 #include<cstring> 4 #include<algorithm> 5 using namespace std; 6 const int MAXN=10000000+50; 7 typedef long long ll; 8 int n; 9 int prime[MAXN],pnum=0; 10 int phi[MAXN]; 11 ll sum[MAXN]; 12 13 void get_phi(int maxn) 14 { 15 memset(phi,0,sizeof(phi)); 16 memset(sum,0,sizeof(sum)); 17 phi[1]=sum[1]=1; 18 for (int i=2;i<=maxn;i++) 19 { 20 if (phi[i]==0) 21 { 22 prime[++pnum]=i; 23 phi[i]=i-1; 24 } 25 for (int j=1;j<=pnum;j++) 26 { 27 if (i*prime[j]>maxn) break; 28 if (i%prime[j]==0) phi[i*prime[j]]=phi[i]*prime[j]; 29 else phi[i*prime[j]]=phi[i]*(prime[j]-1); 30 } 31 sum[i]=sum[i-1]+phi[i]; 32 } 33 } 34 35 int main() 36 { 37 scanf("%d",&n); 38 get_phi(n); 39 ll ans=0; 40 for (int i=1;i<=pnum;i++) 41 { 42 ans+=sum[n/prime[i]]*2-1; 43 } 44 printf("%lld",ans); 45 return 0; 46 }