MFC 判断三个点是否共线

BOOL TheThreePointsAreCollinear(CPoint PrePoint, CPoint CurrentPoint, CPoint NextPoint)
{
 BOOL flag=false;
 if(PrePoint.y==CurrentPoint.y && CurrentPoint.y==NextPoint.y)//如果3个点的Y坐标都相等，则它们是共线的
  flag = true;
 else{
  if(PrePoint.x==CurrentPoint.x){
   if(CurrentPoint.x==NextPoint.x)//如果3个点的X坐标都相等，则它们是共线的
    flag=true;
  }
  else{
   if(NextPoint.y-PrePoint.y-(CurrentPoint.y-PrePoint.y)*(NextPoint.x-PrePoint.x)/(CurrentPoint.x-PrePoint.x) == 0)
    flag = true;//如果第三个点的坐标值满足前两个点所确定的直线方程。则这三个点共线
  }  
 } 
 return flag;
}