Integer to English Words

https://leetcode.com/problems/integer-to-english-words/

class Solution {
public:

    vector<string> twenty;
    vector<string> tens;
    vector<string> thousands;

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

        twenty = {"", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine", "Ten", "Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen"};

        tens = {"", "Ten", "Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety"};

        thousands = {"", "Thousand", "Million", "Billion"};

        int i = 0;
        string words;

        while(num > 0)
        {
            if(num%1000 != 0)
            {
                words = helper(num%1000) + thousands[i] + " " + words;
            }
            num = num/1000;
            i++;
        }

        while(words.back() == ' ')
        {
            words.pop_back();
        }

        return words;
    }

    string helper(int num)
    {
        if(num == 0)
        {
            return "";
        }
        else if(num < 20)
        {
            return twenty[num] + " ";
        }
        else if(num < 100)
        {
            return tens[num/10] + " " + helper(num%10);
        }
        else
        {
            return twenty[num/100] + " Hundred " + helper(num%100);
        }
    }
};