99 Scala problems -> problem 15

gistfile1.scala

def duplicateN [X] (n:Int, xs:List[X]) : List[X] = (n, xs) match {
    case (n, xs) if n < 0 => throw new RuntimeException ("illegal op: negative N")
    case (_, Nil)         => xs
    case (0|1, xs)        => xs
    case (_, h::t)        => (h::Nil) ::: copyN(n-1, h::Nil) ::: copyN(n,t)
}