LeetCode 273 Integer to English Words

/*
  Problem: Convert a non-negative integer to its english words representation. Given input is guaranteed to be less than 231 - 1.

Ex.1234567 -> "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

Solution: Basic idea is create a generic method to convert a number less than 1000 to words. Then for all the numbers
greater than 1000, break it into 1000's Ex. how many billions or how many millions and use the same method to 
convert the number into English words
*/
public class IntegerToEnglishWords273 {
    HashMap map = new HashMap();
    public String numberToWords(int num) {
        fillMap();
        StringBuilder sb = new StringBuilder();
        if(num == 0)
            return map.get(0);
        if(num >= 1000000000){
            int numberOfBillion = num /1000000000;
            sb.append(convert(numberOfBillion)).append(" Billion");
            num = num %1000000000;
        }
        if(num >= 1000000){
            int numberOfMillion = num /1000000;
            sb.append(convert(numberOfMillion)).append(" Million");
            num = num %1000000;
        }
        if(num >= 1000){
            int numberOfThousand = num /1000;
            sb.append(convert(numberOfThousand)).append(" Thousand");
            num = num %1000;
        }
        if(num >0)
            sb.append(convert(num));
        return sb.toString().trim();
    }

    /*
    This method takes care of converting a number less than 1,000 to english
    */
    public String convert(int num){
        StringBuilder sb = new StringBuilder();
        if(num >= 100){
            int numHundred = num /100;
            sb.append(" " +map.get(numHundred) +" Hundred");
            num = num %100;
        }
        if(num >0){
            if(num > 0 && num < 20){
                sb.append(" " +map.get(num));
            }else {
                int numTen = num /10;
                sb.append(" " + map.get(numTen*10));

                int numOne = num %10;
                if(numOne >0)
                    sb.append(" " + map.get(numOne));
            }
        }
        return sb.toString();
    }

    public void fillMap(){
        map.put(0, "Zero");
        map.put(1, "One");
        map.put(2, "Two");
        map.put(3, "Three");
        map.put(4, "Four");
        map.put(5, "Five");
        map.put(6, "Six");
        map.put(7, "Seven");
        map.put(8, "Eight");
        map.put(9, "Nine");
        map.put(10, "Ten");
        map.put(11, "Eleven");
        map.put(12, "Twelve");
        map.put(13, "Thirteen");
        map.put(14, "Fourteen");
        map.put(15, "Fifteen");
        map.put(16, "Sixteen");
        map.put(17, "Seventeen");
        map.put(18, "Eighteen");
        map.put(19, "Nineteen");
        map.put(20, "Twenty");
        map.put(30, "Thirty");
        map.put(40, "Forty");
        map.put(50, "Fifty");
        map.put(60, "Sixty");
        map.put(70, "Seventy");
        map.put(80, "Eighty");
        map.put(90, "Ninety");
    }
}