可変引数Rangeの重複しない整数の個数を求める。

可変引数Rangeの重複しない整数の個数を求める。アルゴリズムは＜０(n＊n)がいいなと思ったけど…

def cover(rs: Range*):Int = ???


assert(cover(0 to 4, 1 to 5) == 6)
assert(cover(0 to 4, 1 to 3) == 5)
assert(cover(0 to 4, 6 to 7) == 7)
assert(cover(0 to 4, 6 to 7, 2 to 6) == 8)

下記以外のがあれば、教えていただきたいです。

def cover(rs: Range*) = rs.map(_.toList).flatMap(x => x).distinct.size

def cover(rs: Range*): Int = {
  @annotation.tailrec
  def helper(xs: List[Range], accu: Int):Int = xs match {
    case Nil => accu
    case r :: Nil => accu + r.size
    case r1 :: r2 :: tail => 
      val nextAccu = accu + r1.size
      if (r2.head > r1.last) helper(r2 :: tail, nextAccu)
      else if (r2.last > r1.last) helper(((r1.last + 1) to r2.last) :: tail, nextAccu)
      else helper(r1 :: tail, accu)
  }
  helper(rs.sortBy(_.start).toList, 0)
}