create a binary tree
node root = new node ( 20 ) ; root . left = new node ( 10 ) ; root . right = new node ( 30 ) ; root . left . left = new node ( 5 ) ; root . left . right = new node ( 15 ) ; root . left . right . left = new node ( 12 ) ; root . right . left = new node ( 25 ) ; root . right . right = new node ( 40 ) ;