POJ1584 判断多边形是否为凸多边形，并判断点到直线的距离

求点到直线的距离：

double dis(point p1,point p2)
{
   if(fabs(p1.x-p2.x)<exp)//相等的
  {
    return fabs(p2.x-pegx);
    }
  else
     {
   double k=(p2.y-p1.y)/(p2.x-p1.x);
   double b=p2.y-k*p2.x;
   return fabs(k*pegx-pegy+b)/sqrt(k*k+1);//返回的是距离的
   }
}
判断多边形是否为凸多边形

if the hole contains any protrusions (i.e., if there exist any two interior points in the hole that, if connected by a line segment, that segment would cross one or more edges of the hole)（括号里的是凸多边形定义的英文描述）, then the filled-in-hole would not be structurally sound enough to support the peg under normal stress as the furniture is used.

按顺序读入边，边和边是按照一个顺序转的（叉积全为正或者全为负）