gistfile1.txt

data Tree a = Empty | Node a (Tree a) (Tree a)
    deriving Show

type IntTree = Tree Int

collapse :: IntTree -> [Int]
collapse t =
    case t of
         Empty -> [-1]
         Node a Empty   r     -> a : -1 : collapse r
         Node a l       Empty -> a : (collapse l ++ [-1])
         Node a l       r     -> a : collapse l ++ collapse r

main = 
    --         1
    --          \
    --           2
    --         /   \
    --        3     4
    --               \
    --                5
    --
    let tree :: Tree Int = Node 1 (Empty) (Node 2 (Node 3 Empty Empty) (Node 4 Empty (Node 5 Empty Empty)))
    in
        print $ collapse tree
        -- Prints: [1,-1,2,3,-1,-1,4,-1,5,-1,-1]