import Data.Array (Array, elems)
import Data.Array.ST (runSTArray, newArray, writeArray)

minfreeArray2 :: [Int] -> Int
minfreeArray2 = search . checklist

search :: Array Int Bool -> Int
search = length . takeWhile id . elems

checklist :: [Int] -> Array Int Bool
checklist xs = runSTArray $ do
  a <- newArray (0, n) False
  sequence [writeArray a x True | x <- xs, x < n]
  return a
  where
  n = length xs