4、欧拉函数 &

AcWing 873. 欧拉函数 - AcWing

AcWing 874. 筛法求欧拉函数

#include <iostream>

using namespace std;


int phi(int x)
{
    int res = x;
    for (int i = 2; i <= x / i; i ++ )
        if (x % i == 0)
        {
            res = res / i * (i - 1);
            while (x % i == 0) x /= i;
        }
    if (x > 1) res = res / x * (x - 1);

    return res;
}


int main()
{
    int n;
    cin >> n;
    while (n -- )
    {
        int x;
        cin >> x;
        cout << phi(x) << endl;
    }

    return 0;
}

#include <iostream>

using namespace std;

typedef long long LL;

const int N = 1000010;


int primes[N], cnt;
int euler[N];
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] = euler[i] * primes[j];
                break;
            }
            euler[t] = euler[i] * (primes[j] - 1);
        }
    }
}


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;
}