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"

public String numberToWords(int num) {
        if (num == 0) return "Zero";
        String res = "";
        String[] units = new String[]{"", "Thousand", "Million", "Billion"};
        int i = 0;
        while (num > 0) {
            if (num % 1000 != 0) {
                res = helper(num % 1000) + units[i] + " " + res;
            }
            num /= 1000;
            i++;
        }
        return res.trim();
    }
    
    private final String[] lessThan20 = new String[]{"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
    private final String[] tens = new String[]{"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty",