data BinTree a = Leaf | Node a (BinTree a) (BinTree a)

collapse :: BinTree a -> (a -> b -> b -> b) -> b -> b
collapse (Leaf) _ x = x
collapse (Node a left right) f x = f a (collapse left f x) (collapse right f x)

treeToList :: BinTree a -> [a]
treeToList t = collapse t helper []
    where
        helper x y z = [x] ++ y ++ z