segment problem

segment_problem.scala

case class RoadSegment(id:Int, segType:String)
case class Leg(begin:RoadSegment, end:Option[RoadSegment])

val segments = Seq(
  RoadSegment(1, "D"),
  RoadSegment(2, "D"),
  RoadSegment(3, "D"),
  RoadSegment(4, "W"),
  RoadSegment(5, "B"),
  RoadSegment(6, "B"),
  RoadSegment(7, "W")
)

// 1. Close to what I'd conceived in the interview
segments.foldLeft(List[Leg]()) { (acc, seg) =>
  if (acc == Nil) {
    acc ++ List(Leg(seg, None))
  } else {
    if (acc.head.begin.segType == seg.segType) {
      Leg(acc.head.begin, Some(seg)) :: acc.tail
    } else {
      Leg(seg, None) :: acc
    }
  }
}.reverse

// 2. With less logic for nil
segments.tail.foldLeft(List[Leg](Leg(segments.head, None))) { (acc, seg) =>
  if (acc.head.begin.segType == seg.segType) {
    Leg(acc.head.begin, Some(seg)) :: acc.tail
  } else {
    Leg(seg, None) :: acc
  }
}.reverse