[Conditional-Statements] 11. Numbers As Words

using System;

class NumberAsWords
{
    static void Main()
    {
        Console.Write("Please enter a number (0-999): ");
        int number = int.Parse(Console.ReadLine());

        string[] unitsMap = {"Zero", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve",
                            "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        string[] tensMap = { "Zero", "Ten", "Twenty", "Thirty", "Fourty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };
        string words = null;

        int hundreds = number / 100;
        int tens = number / 10 % 10;
        int units = number % 10;

        bool isZero = hundreds == 0 && tens == 0 && units == 0;

        if (number < 0 || number > 999)
        {
            Console.WriteLine("Invalid number. Enter a number from 0 to 999.");
        }
        else
        {
            if (isZero)
            {
                words += unitsMap[units];
            }
            if (hundreds != 0 && tens >= 0 && units != 0)
            {
                words += unitsMap[hundreds] + " hundred and ";
            }
            else if (hundreds != 0 && tens >= 0 && units == 0)
            {
                words += unitsMap[hundreds] + " hundred and ";
            }
            else if (hundreds != 0 && tens == 0)
            {
                words += unitsMap[hundreds] + " hundred ";
            }
            if (tens != 0)
            {
                if (tens == 1)
                {
                    words += unitsMap[units + 10] + " ";
                }
                else
                {
                    words += tensMap[tens] + " ";
                    if (units != 0)
                    {
                        words += unitsMap[units];
                    }
                }
            }
            if (units != 0 && tens == 0)
            {
                words += unitsMap[units];
            }
            Console.WriteLine(words);
        }
    }
}