筛法求欧拉函数

题目：

#include <bits/stdc++.h> 
using namespace std;

typedef long long LL;

const int N = 1000010;
int primes[N], cnt; 
int phi[N]; 
bool st[N]; 

LL get_eulers(int n)
{
    phi[1] = 1;
    for (int i = 2; i <= n; i ++) 
    {
        if (!st[i]) 
        {
            primes[cnt ++] = i;
            phi[i] = i - 1;
        }
        for (int j = 0; primes[j] <= n / i; j ++)
        {
            st[primes[j] * i] = true;
            if (i % primes[j] == 0)
            {
                phi[primes[j] * i] = phi[i] * primes[j];
                break;
            }
            phi[primes[j] * i] = phi[i] * (primes[j] - 1);
        }
    }
    LL res = 0;
    for (int i = 1; i <= n; i ++)
        res += phi[i];
    return res;
}

int main()
{
    int n;
    cin >> n;

    cout << get_eulers(n) << endl;
    return 0;
}