判断点是否在多边形内

typedef vector<Point> Polygon;
int isPointInPolygon(Point p,Polygon poly)
{
    int n  = poly.size();
    int wn = 0;
    for(int i = 0;i < n; ++i)
    {
        if(Onsegment(p,poly[i],poly[(i+1)%n])) return -1;
        int k = dcmp(Cross(poly[(i+1)%n]-poly[i],p-poly[i]));
        int d1 = dcmp(poly[i].y-p.y);
        int d2 = dcmp(poly[(i+1)%n].y-p.y);
        if(k>0&&d1 <= 0&&d2 > 0) wn ++;
        if(k<0&&d2 <= 0&&d1 > 0) wn --;
    }
    if(wn != 0)  return 1;
    return 0;
}