20-164853.scala

// スタックにすべて積んで、List("1", "+", "2")みたいな感じになったら、演算する
def ifn(str: String): Int = ifnPrime(str.split(" ").toList, Nil)

def ifnPrime(str: List[String], stack: List[String]): Int = (str, stack) match {
  case ("+"::t, xs) => ifnPrime(t, xs :+ "+")
  case (  n::t, zs) => {
    val list = zs :+ n
    list match {
      case (x::op::y) if op == "+" => ifnPrime(t, List((x.toInt + y.head.toInt).toString))
      case x => ifnPrime(t, x)
    }
  }
  case _ => stack.head.toInt
}

println(ifn("1"))
println(ifn("1 + 2"))
println(ifn("1 + 2 + 3 + 4"))