Hi Tom, Tom Hawkins wrote:

Folding was my first approach:

leaves :: Tree -> Set Int leaves tree = accumLeaves Set.empty tree

accumLeaves :: Set Int -> Tree -> Set Int accumLeaves set (Leaf n) = insert n set accumLeaves set (Branch l r) = foldl accumLeaves set [l,r]

However, with this approach I quickly ran out of stack space. I found this odd, since I thought this program was tail recursive and shouldn't be using the stack.

This is a problem of tail recursion and lazy evaluation not playing nicely. See this page: http://www.haskell.org/haskellwiki/Stack_overflow

My next attempt was to use some form of memorization. Since many of the branches in my trees are equal, memorization should prevent following branches that have already been calculated. My question is, what is the best way to transform the original function to incorporate memorization?

I think something like this could be done, if there's some invariants maintained by the data structure. Is there any additional structure you're imposing beyond that required for the "data" line? -Chad