Merge sorted sublists into sorted list

mergelists.hs

merge []         ys                   = ys
merge xs         []                   = xs
merge xs@(x:xt) ys@(y:yt) | x <= y    = x : merge xt ys
                          | otherwise = y : merge xs yt

mergeLists [] = []
mergeLists [x] = [x]
mergeLists (x:y:rest) = mergeLists $ (merge x y) : mergeLists rest

ll = [[1,2,3], [1,3,6], [1, 10], [3,3,3], [1], [1,1,1,5]]

main = print $ head $ mergeLists ll