maximos :: Ord a => [a] -> [a]

maximos [1,-3,5,2,3,4,7,6,7]                         ==  [1,5,7]
maximos "bafcdegag"                                  ==  "bfg"
maximos (concat (replicate (10^6) "adxbcde")++"yz")  ==  "adxyz"
length (maximos [1..10^6])                           ==  1000000

import Data.List (inits, nub)

-- 1ª definición (por comprensión)
maximos1 :: Ord a => [a] -> [a]
maximos1 xs =
    [x | (ys,x) <- zip (inits xs) xs, all (<x) ys]

-- 2ª definición (por recursión)
maximos2 :: Ord a => [a] -> [a]
maximos2 [] = []
maximos2 (x:xs) = x : maximos2 (filter (>x) xs)

-- 3ª definición (por recursión con acumulador)
maximos3 :: Ord a => [a] -> [a]
maximos3 [] = []
maximos3 (x:xs) = aux xs [x] x
    where aux [] zs _ = reverse zs
          aux (y:ys) zs m | y > m     = aux ys (y:zs) y
                          | otherwise = aux ys zs m

-- 4ª definición (con scanl1):
maximos4 :: Ord a => [a] -> [a]
maximos4 = nub . scanl1 max

-- Comparación de eficiencia
--    λ> maximos1 (concat (replicate (10^3) "adxbcde") ++ "yz")
--    "adxyz"
--    (5.82 secs, 2859603952 bytes)
--    λ> maximos2 (concat (replicate (10^3) "adxbcde") ++ "yz")
--    "adxyz"
--    (0.02 secs, 5879664 bytes)
--    λ> maximos3 (concat (replicate (10^3) "adxbcde") ++ "yz")
--    "adxyz"
--    (0.03 secs, 4153680 bytes)
--    λ> maximos4 (concat (replicate (10^3) "adxbcde") ++ "yz")
--    "adxyz"
--    (0.02 secs, 4163296 bytes)
--
--    λ> maximos2 (concat (replicate (10^6) "adxbcde") ++ "yz")
--    "adxyz"
--    (3.64 secs, 1314485320 bytes)
--    λ> maximos3 (concat (replicate (10^6) "adxbcde") ++ "yz")
--    "adxyz"
--    (6.59 secs, 1706434544 bytes)
--    λ> maximos4 (concat (replicate (10^6) "adxbcde") ++ "yz")
--    "adxyz"
--    (0.89 secs, 1594567000 bytes)
--
--    λ> length (maximos2 [1..10^4])
--    10000
--    (17.34 secs, 4302913816 bytes)
--    λ> length (maximos3 [1..10^4])
--    10000
--    (0.03 secs, 6602488 bytes)
--    λ> length (maximos4 [1..10^4])
--    10000
--    (1.37 secs, 6820408 bytes)