线性筛法求欧拉函数-y总🐂

#include <iostream>
#include <cstring>
#include <algorithm>
using namespace std;
const int N = 1000010;
typedef long long LL;
int primes[N], cnt;
int phi[N];
int 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;
}