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 static String numberToWords(int num) {
        if (num == 0) return "Zero";

        StringBuilder builder = new StringBuilder();
        String[] digits = {"", "One", "Two", "Three",
                "Four", "Five", "Six", "Seven", "Eight", "Nine"};
        String[] teens = {"Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen",
                "Seventeen", "Eighteen", "Nineteen"};
        String[] tens = {"Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};
        String[] units = {"", "Thousand", "Million", "Billion"};

        int unitIndex = 0;
        while (num > 0) {
            int digit = num % 1000;
            if (digit != 0) {
                if (unitIndex > 0) builder.insert(0, units[unitIndex] + " ");
                int ten =