HDU 5784 How Many Triangles(极角排序)
HDU 5733 tetrahedron(三维几何?公式题。。)
HDU 5299 Circles Game (圆的扫描线、树上sg)
HDU 4643 GSM(中垂线)
Hihocoder 1273 清理海报(判断矩形相交)