POJ 1696 Space Ant 极角排序

题意：起点为纵坐标最小的点，每次只能左转，输出点的id

思路：每次按照最新的点进行极角排序

#include <iostream>
#include <cstring>
#include <string>
#include <cmath>
#include <algorithm>
#include <cstdio>
using namespace std;
#define eps 1e-8
double pi=2*asin(1.0);
struct point
{
    int id;
    double x,y;
    point()
    {

    }
    point(double xx,double yy)
    {
        x=xx;
        y=yy;
    }
} p[105],C,D;
double dist(point a,point b)
{
    return hypot(fabs(a.x-b.x),fabs(a.y-b.y));
}
double cross(point a,point b,point c)
{
    return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);
}
bool cmp(point a,point b)
{
    double tp=cross(C,a,b);
    if(fabs(tp)<eps) return 0;
    else if(tp<0) return 0;
    return 1;
}
int main()
{
    int T,i,j,k,m,n;
    scanf("%d",&T);
    while(T--)
    {
        scanf("%d",&n);
        for(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) swap(p[0],p[i]);
            else if(p[i].y==p[0].y) swap(p[0],p[i]);
        }
        for(i=1; i<n; i++)
        {
            C=p[i-1];
            sort(p+i,p+n,cmp);
        }
        printf("%d",n);
        for(i=0; i<n; i++)
            printf(" %d",p[i].id);
        printf("
");
    }
}