public class Solution {
    private static readonly List<(int value, string word)> NUMBERS = new List<(int value, string word)>() {
        (1000000000, "Billion"),
        (1000000, "Million"),
        (1000, "Thousand"),
        (100, "Hundred"),
        (90, "Ninety"),
        (80, "Eighty"),
        (70, "Seventy"),
        (60, "Sixty"),
        (50, "Fifty"),
        (40, "Forty"),
        (30, "Thirty"),
        (20, "Twenty"),
        (19, "Nineteen"),
        (18, "Eighteen"),
        (17, "Seventeen"),
        (16, "Sixteen"),
        (15, "Fifteen"),
        (14, "Fourteen"),
        (13, "Thirteen"),
        (12, "Twelve"),
        (11, "Eleven"),
        (10, "Ten"),
        (9, "Nine"),
        (8, "Eight"),
        (7, "Seven"),
        (6, "Six"),
        (5, "Five"),
        (4, "Four"),
        (3, "Three"),
        (2, "Two"),
        (1, "One"),
    };

    public string NumberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }

        foreach (var (value, word) in NUMBERS) {
            if (num >= value) {
                var prefix = (num >= 100) ? NumberToWords(num / value) + " " : "";
                var suffix = (num % value == 0) ? "" : " " + NumberToWords(num % value);
                return prefix + word + suffix;
            }
        }

        return "";
    }
}