bzoj 2190: [SDOI2008]仪仗队

 1 #include<cstdio>
 2 #include<iostream>
 3 #include<cstring>
 4 #define M 40005
 5 using namespace std;
 6 int ans,n,phi[M],q[M],tot,f[M];
 7 int main()
 8 {
 9     scanf("%d",&n);
10     phi[1]=1;
11     for(int i=2;i<n;i++)
12       {
13         if(!f[i])
14           {
15             tot++;
16             q[tot]=i;
17             phi[i]=i-1;
18             }
19         for(int j=1;j<=tot;j++)
20           {
21             if(q[j]*i>n)
22               break;
23             f[q[j]*i]=1;
24             if(i%q[j])
25               phi[i*q[j]]=phi[i]*(q[j]-1);
26             else
27               phi[i*q[j]]=phi[i]*q[j];
28           }
29       }
30     for(int i=1;i<n;i++)
31       ans+=phi[i];
32     printf("%d\n",2*ans+1);
33 }