C++ 代码

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstdio>
using namespace std;
const int N = 1e6 + 10;

int primes[N], phi[N];
bool str[N];
int n;

void get_ola(int n)
{
    phi[1] = 1;
    int cnt = 0;

    //线性筛
    for(int i = 2; i <= n; i++)
    {
        if(!str[i])
        {
            primes[cnt++] = i;

            //质数的欧拉数为：i-1（1~~i-1都与i互质）
            phi[i] = i - 1;
        }
        for(int j = 0; primes[j] <= n/i; j++)
        {
            str[primes[j] * i] = true;

            int t = primes[j] * i;
            //primes[j] 为i的质数时，primes[j]也是primes[j] * i的质数，所以phi[primes[j] * i] = phi[i] * primes[j];
            if(i % primes[j] == 0)
            {
                phi[t] = phi[i] * primes[j];
                break;
            }
            //primes[j] 不为 i的质因子时
            phi[t] = phi[i] * (primes[j] - 1);
        }
    }

}



int main()
{
    cin >> n;

    get_ola(n);
    long long ans = 0;
    for(int i = 1; i <= n; i++)
        ans += phi[i];

    cout << ans <<endl;

    return 0;
}