Turns Numbers Into English Text 0 - 999999999

#include <iostream>
#include <string>
using namespace std;

int userInput = 0;
int hundredmillion, tenmillion, million,
    hundredthousand, tenthousand, thousand,
    hundred, dozen;

void NumberToText(int);
string PrintNumber(), MillionGroups(), OneThousandGroups(), HundredGroups(int), TenGroups(int);
string BasicNumbers(int), DozensNumbers(int);

int main() {
    cout << "Enter a number between 0 - 999999999: ";
    cin >> userInput;
    if (userInput < 0 || userInput > 999999999) {
        cout << "Please enter correct number!\n" << endl;
        return 0;
    }
    NumberToText(userInput);
    cout << PrintNumber() << endl << endl;
}

string PrintNumber() {
    string text = "";
    text = MillionGroups() + OneThousandGroups() + HundredGroups(hundred) + TenGroups(dozen) + " " + BasicNumbers(userInput);
    return text;
}

string MillionGroups() {
    string milliongroup = "";
    if (hundredmillion != 0) milliongroup += HundredGroups(hundredmillion);
    if (tenmillion != 0) milliongroup += TenGroups(tenmillion) + " ";
    if (million != 0) milliongroup += BasicNumbers(million) + " ";
    if (hundredmillion != 0 || tenmillion != 0 || million != 0)
        return milliongroup + "Million ";
    return "";
}

string OneThousandGroups() {
    string onethousandgroup = "";
    if (hundredthousand != 0) onethousandgroup += HundredGroups(hundredthousand);
    if (tenthousand != 0) onethousandgroup += TenGroups(tenthousand) + " ";
    if (thousand != 0) onethousandgroup += BasicNumbers(thousand) + " ";
    if (hundredthousand != 0 || tenthousand != 0 || thousand != 0)
        return onethousandgroup + "Thousand ";
    return "";
}

string HundredGroups(int inputHundred) {
    if (inputHundred != 0)
        return BasicNumbers(inputHundred) + " Hundred ";
    return "";
}

string TenGroups(int inputTen) {
    if (inputTen >= 1)
        return DozensNumbers(inputTen);
    return BasicNumbers(inputTen);
}

void NumberToText(int inputNumber) {
    if (inputNumber == 0) cout << "Zero" << endl;
    if (BasicNumbers(inputNumber) != "") {
        cout << BasicNumbers(inputNumber);
        inputNumber = 0;
    }
    while (inputNumber >= 100000000) {inputNumber -= 100000000;
        hundredmillion++;}
    while (inputNumber >= 10000000) {inputNumber -= 10000000;
        tenmillion++;}
    while (inputNumber >= 1000000) {inputNumber -= 1000000;
        million++;}
    while (inputNumber >= 100000) {inputNumber -= 100000;
        hundredthousand++;}
    while (inputNumber >= 10000) {inputNumber -= 10000;
        tenthousand++;}
    while (inputNumber >= 1000) {inputNumber -= 1000;
        thousand++;}
    while (inputNumber >= 100) {inputNumber -= 100;
        hundred++;}
    while (inputNumber >= 10) {inputNumber -= 10;
        dozen++;}
    userInput = inputNumber;
}

string BasicNumbers(int inputNumber) {
    // In case yoú were asking why am I not using switch or array
    // Because it hasn't taught yet, I only use basic functions
    if (inputNumber == 1) { return "One"; }
    else if (inputNumber == 2) { return "Two"; }
    else if (inputNumber == 3) { return "Three"; }
    else if (inputNumber == 4) { return "Four"; }
    else if (inputNumber == 5) { return "Five"; }
    else if (inputNumber == 6) { return "Six"; }
    else if (inputNumber == 7) { return "Seven"; }
    else if (inputNumber == 8) { return "Eight"; }
    else if (inputNumber == 9) { return "Nine"; }
    else if (inputNumber == 10) { return "Ten"; }
    else if (inputNumber == 11) { return "Eleven"; }
    else if (inputNumber == 12) { return "Twelve"; }
    else if (inputNumber == 13) { return "Thirteen"; }
    else if (inputNumber == 14) { return "Fourteen"; }
    else if (inputNumber == 15) { return "Fifteen"; }
    else if (inputNumber == 16) { return "Sixteen"; }
    else if (inputNumber == 17) { return "Seventeen"; }
    else if (inputNumber == 18) { return "Eighteen"; }
    else if (inputNumber == 19) { return "Nineteen"; }
    return "";
}

string DozensNumbers(int inputNumber) {
    //if (inputNumber == 1) { return "Ten"; }
    if (inputNumber == 2) { return "Twenty"; }
    else if (inputNumber == 3) { return "Thirty"; }
    else if (inputNumber == 4) { return "Fourty"; }
    else if (inputNumber == 5) { return "Fifty"; }
    else if (inputNumber == 6) { return "Sixty"; }
    else if (inputNumber == 7) { return "Seventy"; }
    else if (inputNumber == 8) { return "Eightty"; }
    else if (inputNumber == 9) { return "Ninety"; }
    return "";
}