Wannafly Camp 2020 Day 7H 游戏 - 欧拉筛,GCD

忘记特判 \(1\) ，血了一地

听说 \(O(n^2 \log n)\) 能过？

#include <bits/stdc++.h>
#define int long long
using namespace std;
const int MAXN = 5005;
bool isNotPrime[MAXN + 1];
int mu[MAXN + 1], phi[MAXN + 1], primes[MAXN + 1], cnt;
inline void euler()
{
    isNotPrime[0] = isNotPrime[1] = true;
    mu[1] = 1;
    phi[1] = 1;
    for (int i = 2; i <= MAXN; i++)
    {
        if (!isNotPrime[i])
        {
            primes[++cnt] = i;
            mu[i] = -1;
            phi[i] = i - 1;
        }

        for (int j = 1; j <= cnt; j++)
        {
            int t = i * primes[j];
            if (t > MAXN) break;

            isNotPrime[t] = true;

            if (i % primes[j] == 0)
            {
                mu[t] = 0;
                phi[t] = phi[i] * primes[j];
                break;
            }
            else
            {
                mu[t] = -mu[i];
                phi[t] = phi[i] * (primes[j] - 1);
            }
        }
    }
}

int n;

signed main() {
    cin>>n;
    if(n==1) {cout<<"0/1"<<endl; return 0;}
    euler();
    int ans = 0;
    for(int i=2;i<=n;i++) ans += phi[i];
    int p=ans*(n/2)*2, q=n*(n-1);
    int g=__gcd(p,q);
    if(g) p/=g, q/=g;
    cout<<p<<"/"<<q<<endl;
}