RPN.swift

let tokens = "15 7 1 1 + - / 3 * 2 1 1 + + -".split(separator: " ").map { String($0) }

let operators: [String: (Int, Int) -> (Int)] = [
    "+" : (+),
    "-" : (-),
    "*" : (*),
    "/" : (/)
]

let outstack = tokens.reduce([Int]()) { (stack, token) in
    var mutableStack = stack
    if let opr = operators[token] {
        let op2 = mutableStack.popLast()!
        let op1 = mutableStack.popLast()!
        let result = opr(op1, op2)
        mutableStack.append(result)
    } else if let op = Int(token) {
        mutableStack.append(op)
    }
    return mutableStack
}

outstack.last