poj 1696 极角排序（解题报告）

#include<iostream>
#include<cmath>
#include<algorithm>
using namespace std;
double eps=1e-8;
int sgn(double x)
{
    if(fabs(x)<=eps)return 0;
    if(x<0)return -1;
    return 1;
}
struct Point
{
    double x,y;
    int index;
    Point (){}
    Point(double _x,double _y)
    {
        x=_x;
        y=_y;
    }
    Point operator -(const Point &b)const
    {
        return Point(x-b.x,y-b.y);
    }
    double operator *(const Point &b)const
    {
        return x*b.x+y*b.y;
    }
    double operator ^(const Point &b)const
    {
        return x*b.y-y*b.x;
    }
};

double dist(Point a,Point b)
{
    return sqrt((a-b)*(a-b));
}
int pos;
Point p[100];
bool cmp(Point a,Point b)
{
    double tmp=sgn((a-p[pos])^(b-p[pos]));
    if(tmp==0)return dist(a,p[pos])<dist(b,p[pos]);
    if(tmp<0)return false;
    return true;

}

int main ()
{
    int T;
    cin>>T;
    int n;
    while(T--)
    {
        cin>>n;
        for(int i=0;i<n;i++)
        {
            cin>>p[i].index>>p[i].x>>p[i].y;
            if(p[i].y<p[0].y||(p[i].y==p[0].y&&p[i].x<p[0].x))
                swap(p[0],p[i]);
        }
        pos=0;
        for(int i=1;i<n;i++)
        {
            sort(p+i,p+n,cmp);//这里是i,排序i后的点
            pos++;
        }
        cout<<n;
        for(int i=0;i<n;i++)
            cout<<‘ ‘<<p[i].index;
        cout<<endl;
    }

    return 0;

}