Convert roman numerals to integers

roman_numeral.swift

import Foundation

let values: [Character: Int] = [
    "I" : 1,
    "V" : 5,
    "X" : 10,
    "L" : 50,
    "C" : 100,
    "D" : 500,
    "M" : 1000
]

// Translate roman literal to integer
// iterate over chars
// track total + track subtractions to next value

func roman(_ string: String) -> Int? {
    var total = 0
    var lastValue : Int?
    for i in string.uppercased().characters {
        if let current = values[i] {
            if let last = lastValue {
                if current > last {
                    total -= last
                } else {
                    total += last
                }
            }

            lastValue = current

        } else {
            return nil
        }
    }

    total += lastValue ?? 0

    return total
}


roman("x") // 10
roman("ix") // 9
roman("lxiii") // 63
roman("MCMXCIX") // 1999