rose_tree_iterative_walk.hs

treeWalkH :: Tree a -> [a]
treeWalkH EmptyTree = []
treeWalkH (Tree root) = treeWalkH' root

treeWalkH' :: Node a -> [a]
treeWalkH' = reverse . loop [] []
  where
    loop out []     (Node v [])     = v:out -- End of traversal
    loop out stack  (Node v (c:cs)) = loop (v:out) (toStack (cs:stack)) c
    loop out (h:t)  (Node v [])     = loop (v:out) (toStack (tail h:t)) (head h)
    toStack t = dropWhile null t