Problem 26

http://projecteuler.net/index.php?section=problems&id=26

--Haskell (needs -package containers), 20090102

import List (elemIndex)
import Maybe (fromJust)
import Data.Map (findMax, fromList)

search :: Int -> (Int, Int)
search n = (fromJust $ search' Nothing 1 [], n)
  where
    search' :: Maybe Int -> Int -> [Int] -> Maybe Int
    search' _        0 _ = Just 0
    search' (Just k) _ _ = Just (k+1)
    search' Nothing  t l = search' (elemIndex t l) (t*10`mod`n) (t:l)

main = print $ snd $ findMax $ fromList $ map search [2..999]

改善の余地あり．