线筛 初见

#include<iostream>
#include<cstdio>
using namespace std;
int phi[40001]={0},prime[40001]={0};
bool flag[40001]={false};
void work(int n)
{
    int i,j;
    for (i=2;i<=n;++i)
    {
        if (!flag[i])
        {
            ++prime[0];prime[prime[0]]=i;phi[i]=i-1;
        }
        for (j=1;j<=prime[0]&&i*prime[j]<=n;++j)
        {
            flag[prime[j]*i]=true;
            if (i%prime[j]==0)
            {
                phi[i*prime[j]]=phi[i]*prime[j];break;
            }
            else phi[i*prime[j]]=phi[i]*(prime[j]-1);
        }
    }
}
int main()
{
    int n,ans=0,i;
    scanf("%d",&n);
    work(n);
    for (i=2;i<n;++i)
        ans+=phi[i]*2;
    ans+=3;
    if (n==1) printf("1\n");
    else printf("%d\n",ans);
}