273. Integer to English Words

class Solution {
    
    private final String[] LESS_THAN_20 = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private final String[] TENS = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
    private final String[] THOUSANDS = {"", "Thousand", "Million", "Billion"};

    public String numberToWords(int num) {
        if (num == 0) {
            return "Zero";
        }
        StringBuilder sb = new StringBuilder();
        int index = 0;
        while (num != 0) {
            if (num % 1000 != 0) {
                StringBuilder tmp = new StringBuilder();
                helper(tmp, num % 1000);
                sb.insert(0, tmp.append(THOUSANDS[index]).append(" "));
            }
            index++;
            num /= 1000;
        }
        return sb.toString().trim();
    }
    
    private void helper(StringBuilder sb, int num) {
        if (num == 0) {
            return;
        } else if (num < 20) {
            sb.append(LESS_THAN_20[num]).append(" ");
            return;
        } else if (num < 100) {
            sb.append(TENS[num/10]).append(" ");
            helper(sb, num % 10);
        } else {
            sb.append(LESS_THAN_20[num / 100]).append(" Hundred ");
            helper(sb, num % 100);
        }
    }
}