P13.scala

  def encodeDirect[T](ts: Seq[T]): Seq[(Int, T)] = {
    def encode(xs: Seq[T], acc: Seq[(Int, T)]): Seq[(Int, T)] = {
      if (ts.isEmpty) acc
      else {
        val (packed, next) = ts.span(_ == ts.head)
        encode(next, (packed.length, packed.head) +: acc)
      }
    }
    encode(ts, Nil)
  }