模板：四点共面

 1 //返回值为0的时候四点共面 
 2 int is_coplanar(point a,point b,point c,point d){
 3     point ab,ac,ad;
 4     ab.x=a.x-b.x,ab.y=a.y-b.y,ab.z=a.z-b.z;
 5     ac.x=a.x-c.x,ac.y=a.y-c.y,ac.z=a.z-c.z;
 6     ad.x=a.x-d.x,ad.y=a.y-d.y,ad.z=a.z-d.z;
 7     int tmp=ab.x*ac.y*ad.z+ab.y*ac.z*ad.x+ac.x*ad.y*ab.z;
 8     tmp-=ab.z*ac.y*ad.x+ab.x*ac.z*ad.y+ab.y*ac.x*ad.z;
 9     return tmp;
10 }