(*) Construct the bottom-up order sequence of the tree nodes.
Write a predicate bottom_up(Tree,Seq) which constructs the bottom-up sequence of the nodes of the multiway tree Tree.
bottom_up :: Tree a -> [a] bottom_up (Node x ts) = concatMap bottom_up ts ++ [x]
A more efficient version using an accumulator:
bottom_up :: Tree a -> [a] bottom_up t = bottom_up_aux t  where bottom_up_aux :: Tree a -> [a] -> [a] bottom_up_aux (Node x ts) xs = foldr bottom_up_aux (x:xs) ts