Problem 14 Solutions

Solution 1 - recurse

duplicate : List a -> List a
duplicate list =
    case list of
        [] -> 
            []

        x :: xs ->
            x :: x :: duplicate xs

Solution 2 - concatMap

duplicate : List a -> List a
duplicate list =
    List.concatMap (\x -> [x, x]) list

Solution 3 - fold

duplicate : List a -> List a
duplicate list =
    List.foldl (\x y -> y ++ [x, x]) [] list

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