【欧拉函数】AcWing874.筛法求欧拉函数——筛法求欧拉函数

AcWing874.筛法求欧拉函数

题解

#include <iostream>

using namespace std;

typedef long long LL;

const int N =  1e6 + 10;

int primes[N], eulars[N], cnt;
bool st[N];

void get_eulars(int n)
{
    eulars[1] = 1;
    for(int i = 2; i <= n; ++i)
    {
        if(!st[i])
        {
            primes[cnt ++ ] = i;
            eulars[i] = i - 1;
        }
        for(int j = 0; primes[j] <= n / i; ++j)
        {
            int t = i * primes[j];
            st[t] = true;
            if(i % primes[j] == 0)
            {
                eulars[t] = eulars[i] * primes[j];
                break;
            }
            eulars[t] = eulars[i] * (primes[j] - 1);
        }
    }
}

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

    LL res = 0;
    for(int i = 1; i <= n; ++i)
        res += (LL)eulars[i];

    cout << res << endl;
    return 0;
}