两条直线（顺逆时针判断）

struct point
{
    double x,y;
}a[Max];
double chaji(point p1,point p2)
{
    return p1.x*p2.y-p1.y*p2.x;
}
double dot(point p1,point p2)
{
    return p1.x*p2.x-p1.y*p2.y;
}
int ccw(point p0,point p1,point p2)
{
    p1.x=p1.x-p0.x,p1.y=p1.y-p0.y;
    p2.x=p2.x-p0.x,p2.y=p2.y-p0.y;
    if(chaji(p1,p2)>0) return 1;  //逆时针
    if(chaji(p1,p2)<0) return 2;  //顺时针
    if(dot(p1,p2)<0) return 3;   // 两条直线共线且相反
    if(p1.x*p1.x+p1.y*p1.y<p2.x*p2.x+p2.y*p2.y) return 4;  //方向 p0->p1->p2
    return 5;   //p2在中间 

}

原文地址：https://www.cnblogs.com/ww123/p/9038516.html