20-125904.scala

def pn(str: String): Int = pnPrime(str.split(" ").toList)._1

// 一意型パターン
def pnPrime(src: List[String]): (Int, List[String]) = src match {
  case "+"::xs => {
    val (a, n1) = pnPrime(xs)
    val (b, n2) = pnPrime(n1)
    (a.toInt + b, n2)
  }
  case _ => (src.head.toInt, src.tail)
}

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