99 questions/Solutions/12

(**) Decode a run-length encoded list.

Given a run-length code list generated as specified in problem 11. Construct its uncompressed version.

decodeModified :: [ListItem a] -> [a]
decodeModified = concatMap decodeHelper
    where
      decodeHelper (Single x)     = [x]
      decodeHelper (Multiple n x) = replicate n x

We only need to map single instances of an element to a list containing only one element and multiple ones to a list containing the specified number of elements and concatenate these lists.

A solution for the simpler encoding from problem 10 can be given as:

decode :: [(Int, a)] -> [a]
decode = concatMap (uncurry replicate)

This can be easily extended given a helper function:

toTuple :: ListItem a -> (Int, a)
toTuple (Single x) = (1, x)
toTuple (Multiple n x) = (n, x)

as:

decode :: [ListItem a] -> [a]
decode = concatMap (uncurry replicate . toTuple)

foldl can also be used to solve this problem:

decode :: [ListItem a] -> [a]
decode = foldl (\acc e -> case e of Single x -> acc ++ [x]; Multiple n x -> acc ++ replicate n x) []