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) {
    StringBuilder builder = new StringBuilder();
    if (num == 0) {
      return "Zero";
    }
    String[] unitsName = {"", "Thousand", "Million", "Billion"};
    int unitIndex = 0;
    while (num > 0) {
      int temp = num % 1000;
      if (temp != 0) {
        builder.insert(0, unitsName[unitIndex] + " ");
      }
      builder.insert(0, convertToWords(temp));
      num = num / 1000;
      unitIndex++;
    }
    return builder.toString().trim();
  }
  /**
   * Convert 3 digit number to english words
   * @param num
   * @return
   */
  private String convertToWords(int num) {
    StringBuilder builder = new StringBuilder();
    String[] numNames = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "