1.hs

lastDigit :: [Integer] -> Integer
lastDigit [] = 1
lastDigit [x] = x `rem` 10
lastDigit (x:xs) = (x `rem` 10) ^ y `rem` 10
  where
    y = 
        case xs of
            [k] -> (k - 1) `rem` 4 + 1
            otherwise -> foldr1 f xs
    f a 0 = 1
    f a b =
        let a' =
                if a <= 1
                    then a
                    else (a + 2) `rem` 4 + 2
                in case a' of
                    2 ->
                        if b = 1
                            then 2
                            else 4
                    3 ->
                        if b `rem` 2 = 0
                            then 5
                            else 3
                    otherwise -> a'