import Data.Char

zad2a 0 =  0
zad2a 1 =  5
zad2a n = zad2a (n-1) + 2*(zad2a (n-2))

zad2b 0 =  0
zad2b 1 =  5
zad2b n = f n 0 5 0
    where f k x0 x1 acc
        | k==1 = acc
        | otherwise = f (k-1) x1 (x1+(2*x0)) (x1+(2*x0))

zad3a x
    | length x < 2 = error "List dosen't have at least 2 elements"
    | otherwise = [head (tail x)] ++ [head x] ++ drop 2 x

zad3b x
    | length x < 2 = error "List dosen't have at least 2 elements"
    | otherwise = [x !! (length x -1)] ++  take (length x -2) (tail x) ++ [head x]

zad3c x
    | length x < 4 = error "List dosen't have at least 4 elements"
    | otherwise = [head x] ++ [x !! (length x -2)] ++ take (length x - 4) (drop 2 x) ++ [x !! 1] ++ [last x]


zad4 list a = f list a 0
    where
    f [] el n = n
    f (x:xs) el n
        | x == el = f xs el (n+1)
        | otherwise = f xs el n

zad5 x y
    | length x /= length y = False
    | otherwise = all (\x -> x== True) (zipWith (==) x y)

zad6 x y
    | length x /= length y = False
    | otherwise = f x y
    where
    f [] b = True
    f a b
        | elem (head a) b = f (tail a) b
        | otherwise = False

wstaw x [] = [x]
wstaw x (y:yx)
    | x > y = y:(wstaw x yx)
    | otherwise = x:y:yx

zad7a x = f x [] (length x)
    where
        f [] acc _ = acc
        f (a:ax) acc x = f ax (wstaw a acc) (x-1)

zad7b x = g x (length x)
    where g e k
        | k == 0 = e
        | otherwise = g (f e [] (length e)) (k-1)
        where
            f [] acc _ = acc
            f (a:ax) acc n
                | n == 1 = acc++[a]
                | a > head ax = f ([a]++tail ax) (acc++[head ax]) (n-1)
                | otherwise = f (ax) (acc++[a]) (n-1)

zad8 x y = f x y []
    where
        f [] b acc = acc++b
        f a [] acc = acc++a
        f (a:ax) (b:bx) acc
            | a < b = f ax (b:bx) (acc++[a])
            | otherwise = f (a:ax) (bx) (acc++[b])


pusta1 :: Eq a => [a] -> Bool
pusta1 x = (x == [])