struct triangle {
     pt a;
     pt b;
     pt c;
   }
   tree structures seem more useful when there is a lot of data to work
   with. they can be used with goals of organisation.
   but you could make a small one too.
   here is an idea: the points are nodes and the lines are edges.
   struct pt {
     int x;
     int y;
     struct pt_list {
       pt *this;
       pt *next;
     } neighbors;
   };