LightOj1285 - Drawing Simple Polygon（连接多边形各点）

#include <stdio.h>
#include <algorithm>
#include <cstring>
#include <cmath>
using namespace std;
const int N = 2010;
const double eps = 1e-6;
const double PI = acos(-1);
struct point
{
    double x, y;
    int Id;
    point(){}
    point(double x, double y) : x(x), y(y) {}
    point friend operator - (const point &p1, const point &p2)///矢量p2p1;
    {
        return point(p1.x-p2.x, p1.y-p2.y);
    }
    double friend operator ^ (const point &p1, const point &p2)///p1×p2;
    {
        return p1.x*p2.y - p1.y*p2.x;
    }
};

point p[N];

double Dist(point p1, point p2)
{
    double dx = p1.x - p2.x, dy = p1.y - p2.y;
    return sqrt(dx*dx + dy*dy);
}
int cmp1(point p1, point p2)///位置排序，找到最下方的；
{
    if(p1.y != p2.y)
        return p1.y < p2.y;
    return p1.x < p2.x;///若有多个下方的找左边的；
}
int cmp2(point p1, point p2)///极角排序；若极角相同，距离近的在前面；
{
    double k = (p1-p[0])^(p2-p[0]);
    if( k>eps || (fabs(k)<eps && Dist(p1, p[0]) < Dist(p2, p[0]) ))
        return 1;
    return 0;
}

int main()
{
    int n, T, t=1;
    scanf("%d", &T);
    while(T--)
    {
        scanf("%d", &n);
        for(int i=0; i<n; i++)
        {
            scanf("%lf %lf", &p[i].x, &p[i].y);
            p[i].Id = i;
        }

        sort(p, p+n, cmp1);///p[0]为最下方靠左的点；
        sort(p+1, p+n, cmp2);///以p[0]为基点，按叉积进行排序；

        int flag = 0;
        for(int i=n-2; i>0; i--)
        {
            if(((p[n-1]-p[0])^(p[i]-p[0]))!=0)
            {
                flag = i;
                break;
            }
        }
        printf("Case %d:\n", t++);
        if(flag==0)
        {
            printf("Impossible\n");
            continue;
        }
        reverse(p+flag+1, p+n);
        for(int i=0; i<n; i++)
            printf("%d%c", p[i].Id, i==n-1?‘\n‘:‘ ‘);
    }
    return 0;
}