nQueens.scala

class nQueens {
  def isSafe(col: Int, queens: List[Int]): Boolean = {
    val rows = queens.length
    val rowsQueens = (rows-1 to 0 by -1).zip(queens)
    rowsQueens.forall{
      case(r, c) => col != c && math.abs(col - c) != rows - r
    }
  }

  def placeQueens(k: Int): Set[List[Int]] =
    if(k == 0) Set(List())
    else for{
      queens <- placeQueens(k-1)
      col <- 0 until n
      if isSafe(col, queens)
    } yield col :: queens

  def show(queens: List[Int]): String = {
    val lines = for (col <- queens.reverse)
      yield Vector.fill(queens.length)(". ").updated(col, "X ").mkString
    "\n" + lines.mkString("\n")
  }
}

object nQueens extends nQueens{
  val temp: Set[List[Int]] = placeQueens(9)
  println("Total generated solutions: "+ temp.size)
  temp.map(show).mkString("\n/")
}

This is a potential solution for solving the nQueens problem using functional programming