object Flatten {

  def flatten[A](xss : List[List[A]]) : List[A] =
    xss match {
      case Nil => List[A]() //Base case <=> end of list
      case head :: tail => { //Can be separated into head and tail
        var xs = flatten(tail) //Find end of tail
        for (i <- (head.length-1 to 0 by -1)) { //Add head to tail
          xs = head(i) :: xs
        }
        xs //return new list
      }
  }

}