在沒有大量查找表的情況下，如何將整數有效地轉換為其書面形式？

<code>string 文字形式 = 整数转文字(21);</code>

<code class="language-c#">public static class HumanFriendlyInteger
{
    static string[] ones = new string[] { "", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine" };
    static string[] teens = new string[] { "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen" };
    static string[] tens = new string[] { "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };
    static string[] thousandsGroups = { "", " Thousand", " Million", " Billion" };

    private static string FriendlyInteger(int n, string leftDigits, int thousands)
    {
        if (n == 0)
        {
            return leftDigits;
        }

        string friendlyInt = leftDigits;

        if (friendlyInt.Length > 0)
        {
            friendlyInt += " ";
        }

        if (n < 100)
        {
            if (n < 20)
            {
                friendlyInt += teens[n - 10];
            }
            else
            {
                friendlyInt += tens[(n / 10) - 2];
                if (n % 10 > 0)
                {
                    friendlyInt += " " + ones[n % 10];
                }
            }
        }
        else
        {
            friendlyInt += ones[n / 100] + " Hundred";
            if (n % 100 > 0)
            {
                friendlyInt += " " + FriendlyInteger(n % 100, "", 0);
            }
        }

        return friendlyInt + thousandsGroups[thousands];
    }

    public static string IntegerToWritten(int n)
    {
        if (n == 0) return "Zero";
        if (n < 0) return "Minus " + IntegerToWritten(-n);

        string result = "";
        int thousands = 0;

        while (n > 0)
        {
            result = FriendlyInteger(n % 1000, result, thousands) + result;
            n /= 1000;
            thousands++;
        }

        return result.Trim();
    }
}</code>