lcs :: (Eq a) => [a] -> [a] -> [a]
lcs [] _ = []
lcs _ [] = []

lcs (x:xs) (y:ys) =
    if (x == y) then x : lcs xs ys
    else longer (lcs xs (y:ys), lcs (x:xs) ys)
    where longer (a, b) = if (length a > length b) then a else b