poj 1696 Space Ant

#include<stdio.h>
#include<algorithm>
using namespace std;
const int Max=55;
struct Point
{
    int x,y,num;
} p[Max],res[Max];
int pos;
int xmult(Point p1,Point p2,Point p0)
{
    return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}
int dis(Point p1,Point p2)
{
    return (p2.x-p1.x)*(p2.x-p1.x)+(p2.y-p1.y)*(p2.y-p1.y);
}
bool cmp(Point p1,Point p2)
{
    int tmp=xmult(p1,p2,p[pos]);
    if(tmp>0)
        return true;
    if(!tmp&&dis(p[pos],p1)<dis(p[pos],p2))
        return true;
    return false;
}
int main()
{
    int _case,n,i;
    scanf("%d",&_case);
    while(_case--)
    {
        scanf("%d",&n);
        for(i=0;i<n;i++)
        {
            scanf("%d%d%d",&p[i].num,&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]);
        }
        for(i=1;i<n;i++)
        {
            pos=i-1;
            sort(p+i,p+n,cmp);
        }
        printf("%d ",n);
        for(i=0;i<n-1;i++)
        printf("%d ",p[i].num);
        printf("%d\n",p[n-1].num);
    }
}