Modifying a recursive string reverse function

I am doing some recursive exercises. The previous one was to make a reverse() function for a string which basically removes the first character and then combines the solution. I managed to do that, here is the source code (the entire source) The current task is to modify this function (the following exercise in the book) by adding a helper function which reverses a substring of the string. At this moment I am stuck at this. It is my understanding that you use helper functions when you need to pass additional arguments or something and this function takes none so I really have no idea how to approach this problem. Help appreciated.

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

    void reverse(string& text)
    {
        if (text.length() == 0)
        {
            return;
        }
        if (text.length() == 1)
        {
            return;
        }
        else
        {
            string firstLetter = text.substr(0,1);
            text = text.substr(1, text.length()-1);
            reverse(text);
            text += firstLetter;
            }
    }
    int main()
    {
        string a = "tyu";
        reverse(a);
        cout << a << endl;
        return 0;
    }

A guy suggested to use parameters, ect, this is my try with it:

#include <iostream>
#include <string>
using namespace std;
//is actually doing the hard work
void reverse1(string& input, int a, int b)
{
        // base case
        if( a >= b)
        {
             return;
        }
        //swap the characters
        char tmp;
        tmp = input[a];
        input[a] = input[b];
        input[b] = tmp;
        //create the boundries for the new substring
        a++;
        b--;
        //call the function again
        reverse1(input, a, b);
}

// sets the parameters and calls the helper function
void strreverse(string& input)
{

    reverse1(input, 0, input.length()-1);
}

int main()
{
    cout << "Please enter the string which you want to be reversed:";
    string a;
    cin >> a;
    strreverse(a);
    cout << a << endl;
    return 0;
}