【筛法】【数论】[POJ 2478]Farey Sequence

就是用筛法处理一下ϕ然后求个前缀和就好了

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <iostream>
using namespace std;
const int MAXN = 1000000;
long long phi[MAXN+20];
int prime[MAXN+10], pcnt;
bool ispri[MAXN+10];
void init(){
    for(int i=2;i<=MAXN;i++){
        if(!ispri[i]){
            phi[i] = i-1;
            prime[pcnt++] = i;
        }
        for(int j=0; j<pcnt; j++){
            if(i * prime[j] > MAXN) break;
            ispri[i*prime[j]] = true;
            if(i % prime[j] == 0){
                phi[i*prime[j]] = phi[i] * prime[j];
                break;
            }
            phi[i*prime[j]] = phi[i] * (prime[j]-1);
        }
    }
    for(int i=3;i<=MAXN;i++)
        phi[i] += phi[i-1];
}

int main(){
    init();
    int n;
    while(~scanf("%d", &n) && n){
        printf("%lld\n", phi[n]);
    }

    return 0;
}