On Sunday 19 July 2009 09:26:14 Heinrich Apfelmus wrote:
Thomas Hartman wrote:
The code below is, I think, n log n, a few seconds on a million + element list.
I wonder if it's possible to get this down to O(N) by using a hashtable implemementation, or other better data structure.
-- findsums locates pairs of integers in a list that add up to a wanted sum.
findsums :: [Int] -> Int -> S.Set (Int,Int) findsums xs wanted = snd . foldl' f (S.empty,S.empty) $ xs where f (candidates,successes) next = if S.member (wanted-next) candidates then (candidates, S.insert (next,wanted-next) successes) else (S.insert next candidates,successes)
Remember that hash tables are actually O(key length) instead of O(1), so I don't think you can break the log n for really large lists this way since the key length increases as well (unless most elements are equal anyway).
Use a trie of hash tables with ~word-sized pieces of key. -- Dr Jon Harrop, Flying Frog Consultancy Ltd. http://www.ffconsultancy.com/?e