Convert a non-negative integer num to its English words representation. Example 1: Input: num = 123 Output: "One Hundred Twenty Three" Example 2: Input: num = 12345 Output: "Twelve Thousand Three Hundred Forty Five" Example 3: Input: num = 1234567 Output: "One Million Two Hundred Thirty Four Thousand Five Hundred Sixty Seven"

class Solution {
    public String numberToWords(int num) {
        if(num == 0) return "Zero";
        String[] units = new String[] {"", "Thousand", "Million", "Billion"};
        StringBuilder sb = new StringBuilder();
        int i = -1;
        while(num > 0) {
            if(num % 1000 != 0) {
                StringBuilder tmp = new StringBuilder();
                helper(num % 1000, tmp);
                tmp.append(units[++i]);
                sb.insert(0, tmp.toString());
            }
            num /= 1000;
        }
        return sb.toString().trim();
    }
    
    private void helper(int num, StringBuilder sb) {
        String[] lessThan20 = new String[] {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "