1 struct point//存储点
 2 {
 3     double x,y;
 4 };
 5 
 6 double cross(double x1,double y1,double x2,double y2)　//计算叉积
 7 {
 8     return (x1*y2-x2*y1);
 9 }
10 
11 double compare(point a,point b,point c)//计算极角
12 {
13     return cross((b.x-a.x),(b.y-a.y),(c.x-a.x),(c.y-a.y));
14 }

1 bool cmp1(point a,point b)
2 {
3     if(atan2(a.y,a.x)!=atan2(b.y,b.x))
4         return atan2(a.y,a.x)<atan2(b.y,b.x);
5     else return a.x<b.x;
6 }

1 bool cmp2(point a,point b) 
2 {
3     point c;//原点
4     c.x = 0;
5     c.y = 0;
6     if(compare(c,a,b)==0)//计算叉积，函数在上面有介绍，如果叉积相等，按照X从小到大排序
7         return a.x<b.x;
8     else return compare(c,a,b)>0;
9 }

 1 int Quadrant(point a)　　//象限排序，注意包含四个坐标轴
 2 {
 3     if(a.x>0&&a.y>=0)  return 1;
 4     if(a.x<=0&&a.y>0)  return 2;
 5     if(a.x<0&&a.y<=0)  return 3;
 6     if(a.x>=0&&a.y<0)  return 4;
 7 }
 8 
 9 
10 bool cmp3(point a,point b)  //先按象限从小到大排序 再按极角从小到大排序
11 {
12     if(Quadrant(a)==Quadrant(b))//返回值就是象限
13         return cmp1(a,b);
14     else Quadrant(a)<Quadrant(b);
15 }