Converts an integer to its roman numeral representation

Convert digits to roman numeral in Swift.swift

// This updated version should be more efficient due to the constant-time lookup in the dictionary, the use of StringBuilder for efficient string concatenation, and the iteration over the lookup keys in descending order.
func toRomanNumeral(_ num: Int) -> String {
    let lookup: [Int: String] = [
        1000: "M",
        900: "CM",
        500: "D",
        400: "CD",
        100: "C",
        90: "XC",
        50: "L",
        40: "XL",
        10: "X",
        9: "IX",
        5: "V",
        4: "IV",
        1: "I"
    ]

    var result = StringBuilder()
    var remainingNum = num

    for value in lookup.keys.sorted(by: >) {
        while remainingNum >= value {
            result.append(lookup[value]!)
            remainingNum -= value
        }
    }

    return result.toString()
}