euler14.hs

import Data.Array
import Data.List
import Data.Ord (comparing)

main = print $ maximumBy (comparing snd) $ assocs $ collatz 1000000

collatz size = memo
  where
    memo = listArray (1, size) $ 1 : [1 + count n | n <- [2..size]]
    count x
      | x' <= size  = memo ! x'
      | otherwise   = 1 + count x'
      where x'
              | even x = x `div` 2
              | odd  x = 3 * x + 1