UVA 10214 Trees in a Wood

https://vjudge.net/problem/UVA-10214

题意：你站在原点，每个坐标位置有一棵高度相同的树，问能看到多少棵树

ans=Σ gcd（x，y）=1

欧拉函数搞搞

#include<cstdio>
#include<algorithm>
using namespace std;
int phi[2001],p[2001],sum[2001],cnt;
bool v[2001];
int main()
{
    phi[1]=1;
    for(int i=2;i<=2000;i++)
    {
        if(!v[i])
        {
            p[++cnt]=i;
            phi[i]=i-1;
        }
        for(int j=1;j<=cnt;j++)
        {
            if(i*p[j]>2000) continue;
            v[i*p[j]]=true;
            if(i%p[j]) phi[i*p[j]]=phi[i]*(p[j]-1);
            else
            {
                phi[i*p[j]]=phi[i]*p[j];
                break;
            }
        }
    }
    int a,b; long long ans;
    while(scanf("%d%d",&a,&b)!=EOF)
    {
        if(!a) return 0;
        ans=0;
        ans+=b+1;
        for(int i=2;i<=a;i++) 
        {
            ans+=1ll*b/i*phi[i];
            for(int j=b/i*i+1;j<=b;j++) ans+=__gcd(i,j)==1;
        }
        ans<<=2;
        printf("%.7lf
",1.0*ans/(1ll*a*b*4+2*(a+b)));
    }
}