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[] below20 = {"One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};
        
        String[] tens = {"Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        
        String[] thousands = {"Billion", "Million", "Thousand", ""};
        
        int i = 0;
        String words = "";
        
        while(num > 0) {
            if(num % 1000 != 0) {
                words = helper(num % 1000) + thousands[i] + " " + words;
            }
            num = num / 1000;
            i++;
        }