# Poj Solution 1981

```http://poj.org/problem?id=1981

#include<iostream>
#include"math.h"
#include"algorithm"
using namespace std;

struct point
{
double x,y;
}p[310];

double jl(point i,point j)
{
return sqrt((i.x-j.x)*(i.x-j.x)+(i.y-j.y)*(i.y-j.y));
}

bool fzk(point a,point b,double &in,double &out)
{
double jiao,l,x;
if((l=jl(a,b))>2)return 0;

jiao=atan2(b.y-a.y,b.x-a.x);

x=acos(l/2);

in=jiao-x;
out=jiao+x;

return 1;
}

int n,m;
struct node
{
char sign;
double jiao;
}t[310];

int cmp(node a,node b)
{
return a.jiao<b.jiao;
}

void doit()
{
double a,b;

for(i=0;i<n;i++)
{
m=0;
for(j=0;j<n;j++)
{
if(i!=j&&fzk(p[i],p[j],a,b))
{
t[m].jiao=a;t[m].sign=1;m++;
t[m].jiao=b;t[m].sign=2;m++;
}
}

std::sort(&t[0],&t[m],cmp);

now=1;kh=9999;
//    for(j=0;j<m&&t[j].sign==2;j++)
//        ;
if(m)
for(j=0;j<m+kh;j++)
{
if(kh==9999&&t[j%m].sign==2)kh=j+1;
if(t[j%m].sign==1)now++;
else now--;
if(now==0)while(1)
cout<<"error";

}

}

}

int main()
{
int i;
while(1)
{
cin>>n;
if(n==0)break;
for(i=0;i<n;i++)
cin>>p[i].x>>p[i].y;

doit();
}

return 0;
}

```
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