UVA12995 Farey Sequence

UVA12995 Farey Sequence

欧拉函数

同仪仗队那题几乎相同，本质都是求欧拉函数的和

#include<cstdio>
#define N 1000000
int n,cnt,pri[N+2],i,j,t;
long long phi[N+2],s[N+2];
bool v[N+2];
int main(){
    for(i=2;i<=N;++i){
        if(!v[i]) phi[i]=i-1,pri[++cnt]=i;
        for(j=1;j<=cnt;++j){
            t=i*pri[j];
            if(t>N) break;
            v[t]=1;
            if(i%pri[j]) phi[t]=phi[i]*(pri[j]-1);
            else{phi[t]=phi[i]*pri[j];break;}
        }
    }//线性筛欧拉函数板子
    for(i=1;i<=N;++i) s[i]=s[i-1]+phi[i];
    while(scanf("%d",&n)){
        if(!n) break;
        printf("%lld\n",s[n]);
    }return 0;
}