Regular polygon

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数学题，注意二分之前要先排序

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#include <iostream>
#include <cmath>
#include <cstdio>
#include <algorithm>
#include <set>

using namespace std;
const int maxn=555;
struct Point
{
    double x,y;
    Point(){}
    Point(double _x,double _y)
    {
        x=_x;
        y=_y;
    }
    double operator ^(const Point &b)const
    {
        return x*b.y-y*b.x;
    }
    double operator *(const Point &b)const
    {
        return x*b.x+y*b.y;
    }
    Point operator -(const Point &b)const
    {
        return Point(x-b.x,y-b.y);
    }
    bool operator <(const Point &b)const
    {
        if(x!=b.x)
            return x<b.x;
        else
            return y<b.y;
    }
};

Point p[maxn];

int main()
{
    int n;
    while(scanf("%d",&n)!=EOF)
    {
        for(int i=0;i<n;i++)
        {
            scanf("%lf %lf",&p[i].x,&p[i].y);
        }
        int ans=0;
        sort(p,p+n);
        for(int i=0;i<n;i++)
        {
            for(int j=i+1;j<n;j++)
            {
                Point p3,p4;
                p3.x=p[i].x+(p[i].y-p[j].y);
                p3.y=p[i].y+(p[j].x-p[i].x);
                p4.x=p[j].x+(p[i].y-p[j].y);
                p4.y=p[j].y+(p[j].x-p[i].x);
                if(binary_search(p,p+n,p3)==1&&binary_search(p,p+n,p4)==1)
                    ans++;
                p3.x=p[i].x-(p[i].y-p[j].y);
                p3.y=p[i].y-(p[j].x-p[i].x);
                p4.x=p[j].x-(p[i].y-p[j].y);
                p4.y=p[j].y-(p[j].x-p[i].x);
                if(binary_search(p,p+n,p3)==1&&binary_search(p,p+n,p4)==1)
                    ans++;
            }
        }
        cout<<ans/4<<endl;
    }
    return 0;
}