欧拉函数

#include<iostream>
using namespace std;

const int N = 1e6+10;
int primes[N],phi[N],cnt;
bool st[N];

long long get_phi(int n)
{
    phi[1] = 1;
    for(int i=2;i<=n;++i)
    {
        if(!st[i]){//此处一定是质数
            primes[cnt++] = i;
            phi[i] = i - 1;// n * (1 - 1/n)
        }
        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);
        }
    }
}

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

    cout << get_phi(n);
    return 0;
}