Difference between revisions of "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)
```

a naïve solution with `foldl`:

```decode :: [ListItem a]-> [a]
decode = foldl (\x y -> x ++ dh y) []
where
decodeHelper :: ListItem a -> [a]
decodeHelper (Single x)=[x]
decodeHelper (Multiple n x)= replicate n x
```

`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) []
```