227. Basic Calculator II

func calculate(_ s: String) -> Int {
        let arr = Array(s)

        var sum = 0
        var stack = [Int]()

        var i = 0
        var preSign:Character = "+"

        while i < arr.count{
            if arr[i] == " " {
                i += 1
                continue
            }
            if isDigit(arr[i]){
                let start = i
                while i < arr.count && isDigit(arr[i]){
                    i += 1
                }
                let integer = Int(String(arr[start ..< i]))!
                switch preSign{
                    case "+":
                        stack.append(integer)
                    case "-":
                        stack.append( -integer)
                    case "*":
                        stack.append(stack.removeLast() * integer)
                    case "/":
                        stack.append(stack.removeLast() / integer)
                    default:
                        print()
                }
            } else{
                preSign = arr[i]
                i += 1
            }
        }

        while stack.count > 0 {
            sum += stack.removeLast()
        }
        return sum
    }
    func isDigit(_ char: Character) -> Bool{
        if char >= "0" && char <= "9" {return true}
        return false
    }