Space Ant - POJ 1696 (凸包)

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

const int MAXN = 107;
const double EPS = 1e-8;

struct Point
{
    double x, y;
    int id;
    Point(double x=0, double y=0):x(x),y(y){}
    Point operator - (const Point &tmp) const{
        return Point(x-tmp.x, y-tmp.y);
    }
    double operator *(const Point &tmp) const{
        return x*tmp.x + y*tmp.y;
    }
    double operator ^(const Point &tmp) const{
        return x*tmp.y - y*tmp.x;
    }
};
double Dist(Point a, Point b)
{
    return sqrt((a-b)*(a-b));
}
Point p[MAXN];
int ki;

bool cmp(Point a, Point b)
{
    double t = (a-p[ki]) ^ (b-p[ki]);

    if(fabs(t) < EPS)
        return Dist(p[ki], a) < Dist(p[ki], b);
    return t > EPS;
}

int main()
{
    int T;

    scanf("%d", &T);

    while(T--)
    {
        int i, N;

        scanf("%d", &N);

        for(int i=0; i<N; i++)
        {
            scanf("%d%lf%lf", &p[i].id, &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[i], p[0]);
        }

        ki = 0;

        for(i=1; i<N; i++, ki++)
        {
            sort(p+i, p+N, cmp);
        }

        printf("%d", N);
        for(i=0; i<N; i++)
            printf(" %d", p[i].id);
        printf("\n");
    }

    return 0;
}