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[],p[],sum[],cnt;
bool v[];
int main()
{
    phi[]=;
    for(int i=;i<=;i++)
    {
        if(!v[i])
        {
            p[++cnt]=i;
            phi[i]=i-;
        }
        for(int j=;j<=cnt;j++)
        {
            if(i*p[j]>) continue;
            v[i*p[j]]=true;
            if(i%p[j]) phi[i*p[j]]=phi[i]*(p[j]-);
            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 ;
        ans=;
        ans+=b+;
        for(int i=;i<=a;i++)
        {
            ans+=1ll*b/i*phi[i];
            for(int j=b/i*i+;j<=b;j++) ans+=__gcd(i,j)==;
        }
        ans<<=;
        printf("%.7lf\n",1.0*ans/(1ll*a*b*+*(a+b)));
    }
}