JoinListOfLists.scala

  /** 
    * Join list of lists by order of elements
    * val a = List(3, 5, 6)
    * val b = List(1, 2, 7, 8)
    * val expected = List(3, 1, 2, 5, 6, 7, 8)
    *
    * @param lists list of lists of elements
    * @return list of elements
    */
  def join[A](lists: List[List[A]]): List[A] = {
    @annotation.tailrec
    def go[A](ls: List[List[A]], rem: List[List[A]], res: List[A]): List[A] =
      ls match {
        case x :: xs =>
          go(
            xs,
            if (x.isEmpty) rem else (x.tail :: rem),
            if (x.isEmpty) res else x.head :: res
          )
        case Nil if rem.nonEmpty =>
          go(rem, Nil, res)
        case Nil => res
      }

    go(lists, Nil, Nil).reverse
  }