Integer to Words

class Solution {
    public:

        string single_numberToWords(string num) {
            if (num == "1") {
                return "One";
            }
            if (num == "2") {
                return "Two";
            }
            if (num == "3") {
                return "Three";
            }
            if (num == "4") {
                return "Four";
            }
            if (num == "5") {
                return "Five";
            }
            if (num == "6") {
                return "Six";
            }
            if (num == "7") {
                return "Seven";
            }
            if (num == "8") {
                return "Eight";
            }
            if (num == "9") {
                return "Nine";
            }
            return "";
        }

    string double_numberToWords(string num) {
        if (num == "00") {
            return "";
        }
        if (num[0] == '0') {
            return single_numberToWords(num.substr(1, 1));
        }

        if(num == "10"){
            return "Ten";
        }
        if (num == "11") {
            return "Eleven";
        }
        if (num == "12") {
            return "Twelve";
        }
        if (num == "13") {
            return "Thirteen";
        }
        if (num == "14") {
            return "Fourteen";
        }
        if (num == "15") {
            return "Fifteen";
        }
        if (num == "16") {
            return "Sixteen";
        }
        if (num == "17") {
            return "Seventeen";
        }
        if (num == "18") {
            return "Eighteen";
        }
        if (num == "19") {
            return "Nineteen";
        }

        string to_return = "";
        if (num[0] == '2') {
            to_return += "Twenty";
        }
        if (num[0] == '3') {
            to_return += "Thirty";
        }
        if (num[0] == '4') {
            to_return += "Forty";
        }
        if (num[0] == '5') {
            to_return += "Fifty";
        }
        if (num[0] == '6') {
            to_return += "Sixty";
        }
        if (num[0] == '7') {
            to_return += "Seventy";
        }
        if (num[0] == '8') {
            to_return += "Eighty";
        }
        if (num[0] == '9') {
            to_return += "Ninety";
        }

        if (num[1] != '0') {
            to_return += " " + single_numberToWords(num.substr(1, 1));
        }
        return to_return;
    }

    string three_numberToWords(string num) {
        //num is of three digits
        string to_return = "";
        if (num[0] != '0') {
            to_return += single_numberToWords(num.substr(0, 1)) + " " + "Hundred";
        }
        if(!to_return.empty())
            to_return += " " + double_numberToWords(num.substr(1, 2));
        else
            to_return += double_numberToWords(num.substr(1, 2));
        return to_return;
    }

    string denomination(int index) {
        switch (index / 3) {
        case 0:
            return "";
        case 1:
            return " Thousand";
        case 2:
            return " Million";
        case 3:
            return " Billion";
        case 4:
            return " Trillion";
        case 5:
            return " Quadrillion";
        case 6:
            return " Quintillion";
        case 7:
            return " Sextillion";
        case 8:
            return " Septillion";
        case 9:
            return " Octillion";
        case 10:
            return " Nonillion";
        case 11:
            return " Decillion";
        case 12:
            return " Undecillion";
        }

        return "";
    }

    string numberToWords(int numInt) {
        if(numInt == 0) return "Zero";
        string num = "";
        while (numInt != 0) {
            num += to_string(numInt % 10);
            numInt /= 10;
        }
        reverse(num.begin(), num.end());
        cout << num << endl;
        vector < string > answer;
        int i = num.size() - 1;

        int valDeno = 0;
        while (i >= 0) {
            string chunk = "";
            for (int k = 0; k <= 2 && i >= 0; k++) {
                chunk += num[i];
                i--;
                valDeno++;
            }
            reverse(chunk.begin(), chunk.end());
            //now we've got the chunk
            if(chunk.size()==3 && !three_numberToWords(chunk).empty()){
                answer.push_back(three_numberToWords(chunk) + denomination(valDeno - 1));
            }
            else if(chunk.size()==2 && !double_numberToWords(chunk).empty()){
                answer.push_back(double_numberToWords(chunk) + denomination(valDeno - 1));
            }
            else if(chunk.size()==1 && !single_numberToWords(chunk).empty()){
                answer.push_back(single_numberToWords(chunk) + denomination(valDeno - 1));
            }
        }

        string to_return = "";
        for (int i = answer.size() - 1; i >= 0; i--) {
            if(i==0){
                to_return += answer[i];
                break;
            }
            if(answer[i].size()==0) continue;
            to_return += answer[i] + " ";
        }
        string to_return_removeSpace = "";
        for(int i=0; i<to_return.size(); i++){
            if(to_return[i]!=' '){
                to_return_removeSpace += to_return[i];
            }
            else{
                while(i < to_return.size() && to_return[i] == ' '){
                    i++;
                }
                if(i == to_return.size()){
                    break;
                }
                to_return_removeSpace += ' ';

                i--;
            }
        }
        return to_return_removeSpace;
    }
};