线性（欧拉）筛

#include <iostream>
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
using namespace std;
typedef long long ll;
const int maxn = 1e6+5;
bool flag[maxn]; //标记数组
ll phi[maxn]; //欧拉函数值
int prime[maxn]; //同时得到素数筛
int cnt = 0;
void Get_phi(int n)
{
    cnt = 0;
    memset(flag,true,sizeof(flag));
    phi[1] = 1;
    for(int i=2;i<=n;i++)
    {
        if(flag[i]) //素数
        {
            prime[cnt++] = i;
            phi[i] = i-1; //素数的欧拉函数值是i-1
        }
        for(int j=0;j<cnt;j++)
        {
            if(i*prime[j]>n)
            {
                break;
            }
            flag[i*prime[j]] = false;//素数的倍数不是素数
            if(i%prime[j]==0) //i%mod prime = 0,那么phi(i*p) = p*phi(i)
            {
                phi[i*prime[j]] = prime[j]*phi[i];
                break;
            }
            else phi[i*prime[j]] = (prime[j]-1)*phi[i];//i mod prime != 0, 那么 phi(i * prime) == phi(i) * (prime-1)
        }
    }
}
ll sum[maxn];
int main()
{
    Get_phi(1e6);
    for(int i=2;i<=1000;i++)
    {
        sum[i] = sum[i-1]+phi[i];
       // cout<<sum[i]<<endl;
    }
    int n;
    while(cin>>n)
    {
        if(n==0) break;
        cout<<sum[n]<<endl;
    }
    return 0;
}