极角排序

题意:逆时针绕圈,问最长怎么走
https://vjudge.net/problem/POJ-1696

int sgn(double x) {
	if(fabs(x) < eps)return 0;
	else return x<0?-1:1;
}
struct Point {
	double x,y;
	int id;
	Point() {};
	Point (double x,double y):x(x),y(y) {}
	Point operator+(Point B) {
		return Point(x+B.x,y+B.y);
	}
	Point operator-(Point B) {
		return Point(x-B.x,y-B.y);
	}
	Point operator/(double k) {
		return Point(x/k,y/k);
	}
	bool operator == (Point B) {
		return sgn(x-B.x)==0&&sgn(y-B.y)==0;
	}
};

struct Line {
	Point p1,p2;
	Line() {}
	Line(Point p1,Point p2):p1(p1),p2(p2) {}
};

typedef Point Vector;

double Dot(Vector A,Vector B) {
	return A.x*B.x+A.y*B.y;
}

double Cross(Vector A,Vector B) {
	return A.x*B.y-A.y*B.x;
}
double Distance(Point A,Point B) {
	double xx = A.x - B.x;
	double yy = A.y - B.y;
	return sqrt(xx*xx+yy*yy);
}
int n;
int pos;
Point p[N];

bool cmp(Point a,Point b) {
	double t=Cross(a-p[pos],b-p[pos]);
	if(sgn(t)==0)return Distance(p[pos],a)<Distance(p[pos],b);
	else if(sgn(t)<0)return false;
	return true;

}
void work() {
	scanf("%d",&n);
	double x,y;
	for(int i=0; i<n; i++) {   //先找到左下角的点
		scanf("%d%lf%lf",&p[i].id,&p[i].x,&p[i].y);
//		if(p[i].x < p[0].x || (p[i].x == p[0].x
//		                       && p[i].y < p[0].y))swap(p[0],p[i]);
        //注意,先判最下面再判最左边,不然wa
	if(p[i].y < p[0].y || (p[i].y == p[0].y && p[i].x < p[0].x))swap(p[i],p[0]);
	}
	pos =0 ;
	for(int i=1; i<n; i++) {
		sort(p+i,p+n,cmp);
		pos++;
	}
	printf("%d",n);
	for(int i=0;i<n;i++){
		printf(" %d",p[i].id);
	}
	printf("
");
}