18-113337.scala

def solveRPN(input: String): Double = {
  input.split(" ").toList.foldLeft(List[Double]()){(out, in) =>
    in match {
      case "+" => out.dropRight(2) :+ (out.dropRight(1).last + out.last)
      case "-" => out.dropRight(2) :+ (out.dropRight(1).last - out.last)
      case "*" => out.dropRight(2) :+ (out.dropRight(1).last * out.last)
      case "/" => out.dropRight(2) :+ (out.dropRight(1).last / out.last)
      case "%" => out.dropRight(2) :+ (out.dropRight(1).last % out.last)
      case _ => out :+ in.toDouble
    }
  }.head
}

println(solveRPN("1 3 +"))
println(solveRPN("1 3 + 5 2 - *"))