Intersection for ordered lists.

intersect.scala

def intersect(ens: List[Int], ems: List[Int]): List[Int] = (ens, ems) match {
  case (Nil, _) => Nil
  case (_, Nil) => Nil
  case (en :: ensTail, em :: emsTail) if en == em => en :: intersect(ensTail, emsTail)
  case (en :: ensTail, em :: _) if en < em => intersect(ensTail dropWhile { _ < em }, ems)
  case (en :: _, em :: emsTail) => intersect(ens, emsTail dropWhile { _ < en })
}

intersect(List(1, 2, 3, 4), List(3, 4, 5, 6))   // = List(3, 4)
intersect(List(3, 4, 5, 6), List(1, 2, 3, 4))   // = List(3, 4)
intersect(List(1, 2, 3, 4), List(1, 2, 3, 4))   // = List(1, 2, 3, 4)
intersect(List(1, 2, 3, 4), List(5, 6, 7, 8))   // = List()
intersect(List(1, 3, 5, 7), List(2, 4, 6, 8))   // = List()