20-180039.scala

def ifn(str: String): Int = expr(str.split(" ").toList)._1

// expr := term ["+"|"-" expr]
def expr(src: List[String]): (Int, List[String]) = {
  //println(s"[expr] src: $src")
  val (v1, src2) = term(src)
  src2 match {
    case "+"::src3 => {
      val (v2, src4) = expr(src3)
      (v1 + v2, src4)
    }
    case "-"::src3 => {
      val (v2, src4) = expr(src3)
      (v1 - v2, src4)
    }
    case _ => (v1, src2)
  }
}

// term := num ["*"|"/" term]
def term(src: List[String]): (Int, List[String]) = {
  //println(s"[term] src: $src")
  val num = src.head.toInt
  src.tail match {
    case "*"::src2 => {
      val (num2, src3) = term(src2)
      (num * num2, src3)
    }
    case "/"::src2 => {
      val (num2, src3) = term(src2)
      (num / num2, src3)
    }
    case src2 => (num, src2)
  }
}

def test(src: String) = println(src + " = " + ifn(src))

test("1")
test("1 + 2")
test("1 + 2 + 3 + 4")
test("1 * 2 * 3")
test("1 + 2 * 3")
test("1 * 2 + 3")
test("2 * 3 * 4")
test("2 + 3 * 4")
test("2 * 3 + 4")
test("5 - 2")
test("1 - 2 - 3")
test("2 * 3 - 4 * 5 * 6 - 7 * 8")