def matchTwoLists(xs:List[Int], ls: List[Int]): List[Int] = {
  (xs, ls) match  {
    case (Nil, Nil) => Nil
    case (head::tail, Nil) => head :: matchTwoLists(tail, Nil)
    case (Nil, head1::tail1) => head1 :: matchTwoLists(Nil, tail1)
    case (head::tail, head1::tail1) => head :: head1 :: matchTwoLists(tail, tail1)
  }
}