def isLinear(src: List[(Int, Int)]): Boolean = {
  val m = src.toMap.withDefaultValue(-1)
  val tails = m.values.toSet
  src.isEmpty || (src.size == m.size) && {
    src find { case (a,_) => !tails(a) } match {
      case Some((start,_)) =>
        val visited =
          Stream.iterate(start)(m).takeWhile(0 <=).take(src.size+2)
        visited.length == src.size + 1
      case None => false
    }
  }
}