def isOk(solution: List[Int]) = {
      def isOkReq(solution: List[Int], posRestantes: Set[Int]): Boolean = solution match {
        case List() => posRestantes.size == 0
        case x :: xs => {
          val reste = for {
            p <- posRestantes
            if p != x
            d <- List(p + 1, p - 1)
            if d > 0 && d <= nbTerriers
          } yield d
          isOkReq(xs, reste)
        }
      }
      isOkReq(solution, (1 to nbTerriers).toSet)
    }