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 class Solution {
    public String numberToWords(int num) {
        String[] ones = {"", "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 = {"", "Thousand", "Million", "Billion"};
        StringBuilder sb = new StringBuilder();
        int i = 0;
        while (num > 0) {
            if (num % 1000 != 0) {
                sb.insert(0, helper(num % 1000, ones, tens) + thousands[i] + " ");
            }
            num /= 1000;
            i++;
        }
        return sb.length() == 0 ?