线性筛欧拉函数

#include <bits/stdc++.h>
using namespace std;
int P[40005]={1,1},phi[40005];
vector<int> prime;
void getphi(int n){
    for(int i=2;i<=n;i++){
        if(!P[i])prime.push_back(i),phi[i]=i-1;
        for(int j=0;j<prime.size()&&prime[j]*i<=n;j++){
            if(i%prime[j])phi[i*prime[j]]=phi[i]*(prime[j]-1);
            else phi[i*prime[j]]=phi[i]*prime[j];
            P[i*prime[j]]=1;
            if(!i%prime[j])break;
        }
    }
}
int main(){
int n,ans=0;
    cin>>n;
    getphi(n-1);
    return 0;
}