Convert a non-negative integer num to its English words representation.

class Solution {
    public String numberToWords(int num) {
        // check if num is within range
        if(num == 0) return "Zero";
        if(num > 2147483647 || num < 0) return "Invalid Input";
        String[] lessThanTwenty = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        String[] tens = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        String[] thousands = {"", "Thousand", "Million", "Billion"};
        String res = "";
        int i = 0;
        while(num > 0){
            if(num % 1000 != 0)
                res = helper(num%1000, lessThanTwenty, tens)