HDU 2824 The Euler function

筛法计算欧拉函数

#include <iostream>

#include <cstdio>

using namespace std;

const int maxn=3000005;

long long phi[maxn];

int main(){

    int i,j,a,b;

    for(i=1;i<=maxn;i++) phi[i]=i;

    for(i=2;i<=maxn;i+=2) phi[i]/=2;

    for(i=3;i<=maxn;i+=2)if(phi[i]==i){

      for(j=i;j<=maxn;j+=i)

        phi[j]=phi[j]/i*(i-1);

    }

    while(scanf("%d%d",&a,&b)!=EOF){

        long long ans=0;

        for(i=a;i<=b;i++)ans+=phi[i];

        cout<<ans<<endl;

    }

    return 0;

}