[Haskell-cafe] Re: can there be (hash-table using) O(n) version of
this (I think currently) n log n algo?
apfelmus at quantentunnel.de
Sun Jul 19 04:26:14 EDT 2009
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
In any case,
I have this conjecture
On which I will lecture:
A Set of some things
Is sorting for kings.
findsums goal = uncurry sweep . (id &&& reverse) . sort
sweep  _ = 
sweep _  = 
sweep (x:xs) (y:ys) = if x > y then  else
case compare (x+y) goal of
LT -> sweep xs (y:ys)
EQ -> (x,y) : sweep xs ys
GT -> sweep (x:xs) ys
This algorithm needs a proof of correctness, though. And it's longer
that the Data.Set version, too.
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