欧拉函数求质数

#include<iostream>

using namespace std;
typedef long long ll;
const int N = 1e6 + 10;
int euler[N], primes[N], cnt;
bool st[N];
void get_eulers(int n){
    euler[1] = 1;
    for(int i = 2; i <= n; i ++){
        if(!st[i]){
            primes[cnt ++] = i;
            euler[i] = i - 1;
        }
        for(int j = 0; primes[j] <= n / i; j ++){
            int t = primes[j] * i;
            st[t] = true;
            if(i % primes[j] == 0){
                euler[t] = primes[j] * euler[i];
                break;
            }
            euler[t] = (primes[j] - 1) * euler[i];
        }
    }
} 
int main(){
    int n;
    cin >> n;
    get_eulers(n);
    ll res = 0;
    for(int i = 1; i <= n; i ++) res += euler[i];
    cout << res << endl;
    return 0;
}