2023-9-4 筛法求欧拉函数

题目链接：筛法求欧拉函数

#include 
#include 

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])
        {
            phi[i] = i -1;
            primes[cnt++] = i;
        }
        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;
}