[Swift] 문자열 계산하기

import Foundation

func solution(_ my_string:String) -> Int {
    
    var result = [String]()
    
    my_string.components(separatedBy: " ").forEach {
        let str = String($0)
        
        if result.count < 2 {
            result.append(str)
        } else {
            let operation = result.removeLast()
            let num = result.removeLast()
            var tmp = ""
            switch operation {
            case "+" :
                tmp = String(Int(num)! + Int(str)!)
            case "-" :
                tmp = String(Int(num)! - Int(str)!)
            default :
                break
            }
            result.append(tmp)
        }
    }
    
    return Int(result.removeLast())!
}