Given a set of the above nodes:


1  2  3  4

The solution would be :


           b
  +-----------------+
  |           c     |
  |     +-----------+
  +  a  |  b     a  |
  1-----2-----3-----4
  |     c     |     
  +-----------+     

  
  
where a,b,c are edge labels and |{a,b,c}| = 3 = |N|-1