data Tree a = Node a (Tree a) (Tree a) | Tip

flatten :: Tree a -> [a]
flatten x = (flatten' x) []

flatten' :: Tree a -> [a] -> [a]
flatten' (Node x left right) l = (flatten' left (x:(flatten' right l)))
flatten' Tip l = l

main = 
  putStrLn $ show $ flatten $ 
    (Node 2 (Node 1 Tip Tip) (Node 4 (Node 3 Tip Tip) Tip))