Tree and Node design

1) Is it a good decision to model “no node" as `End` object? Or should it be `null` or may be a class?
2) What is the best way to model Nodes with XY position? I decided to wrap node value into object which has coordinates, unlike for example this solution http://aperiodic.net/phil/scala/s-99/p64.scala where they define `PositionedNode` class.
