极角排序模板+判断象限

int xx(point a) //判断象限
{
    if(a.x>0&&a.y>=0)return 1;
    if(a.x<=0&&a.y>0)return 2;
    if(a.x<0&&a.y<=0)return 3;
    if(a.x>=0&&a.y<0)return 4;
}

double cj(node a,node b,node c) //叉积计算
{
    return (a.x-c.x)*(b.y-c.y)-(b.x-c.x)*(a.y-c.y);
}

bool cmp(point a,point b)   //先按象限从小到大排序 再按极角从小到大排序
{
    if(xx(a)==xx(b))return cj(a,b)>0;   //逆时针排序
    else return xx(a)<xx(b);
}