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

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