findLowestCommon.hs

findLowest :: [Int] -> [Int] -> Int
findLowest as bs = findLowest' (sort as) (sort bs)
  where
    findLowest' _ [] = -1
    findLowest' [] _ = -1
    findLowest' (a:ta) (b:tb)
      | a == b = a
      | b < a = findLowest' (a:ta) (tb)
      | otherwise = findLowest' ta (b:tb)