Reconstruct a binary tree from its pre and post order traversals

reconstruct.hs

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

instance (Show a) => Show (Tree a) where
  show NullNode = ""
  show (Node x NullNode NullNode) = show x
  show (Node x l r) = show x ++ "(" ++ show l ++ " " ++ show r ++ ")"

preOrder :: Tree a -> [a]
preOrder NullNode = []
preOrder (Node x l r) = x : (preOrder l ++ preOrder r)

postOrder :: Tree a -> [a]
postOrder NullNode = []
postOrder (Node x l r) = postOrder l ++ postOrder r ++ [x]

inOrder :: Tree a -> [a]
inOrder NullNode = []
inOrder (Node x l r) = inOrder l ++ [x] ++ inOrder r

reconstruct :: (Eq a) => [a] -> [a] -> Tree a
reconstruct pre post
  | length pre == 1 = Node (head pre) NullNode NullNode
  | otherwise = Node (head pre) left right
  where leftRoot = head . tail $ pre
        rightRoot = last . init $ post
        leftPre = takeWhile (/= rightRoot) . tail $ pre
        rightPre = dropWhile (/= rightRoot) . tail $ pre
        leftPost = takeWhile (/= leftRoot) (init post) ++ [leftRoot]
        rightPost = tail . dropWhile (/= leftRoot) . init $ post
        left = reconstruct leftPre leftPost
        right = reconstruct rightPre rightPost

-- Main> reconstruct "iqjhlemvotsbrgyzkca&fpnudwx" "hemljvqsgyrzbtcpudnfw&xakoi"
-- 'i'('q'('j'('h' 'l'('e' 'm')) 'v') 'o'('t'('s' 'b'('r'('g' 'y') 'z')) 'k'('c' 'a'('&'('f'('p' 'n'('u' 'd')) 'w') 'x'))))