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;
}