C# Controlling balanced parenthesis to recursive

Question

I am trying to write a function to control if the parenthesis included in a stringare balanced or not.

I have written the following function:

public bool IsBalanced(string input)
{
    //last condition, gets out
    if (string.IsNullOrEmpty(input)) return true;

    int numOpen = 0;
    bool opened = false;
    foreach (char c in input)
    {
        if (char.ToUpperInvariant(c) =='(')
        {
            opened = true;
            numOpen+=1;
        }
        if (char.ToUpperInvariant(c) == ')')
        {
            if (opened)
            {
                numOpen-=1;
                opened = numOpen > 0;
            }
            else
                return false; //incorrect position parentheses, closes but not opened
        }
    }

    return numOpen == 0;
}

I wanted to change it to a recursive function, but have not been able to do so. Could anyone give a hint how to do it?

Answer 1

The basic idea is to take a variant (numOpen in this case) as an argument. Here is my code:

public bool IsBalancedRec(string input, int numOpen = 0)
{
    if (numOpen < 0)
        return false;
    if (string.IsNullOrEmpty(input))
        return numOpen == 0;

    char c = input[0];
    string rest = input.Substring(1);

    if (c == '(')
        return IsBalancedRec(rest, numOpen + 1);
    else if (c == ')')
        return IsBalancedRec(rest, numOpen - 1);
    else
        return IsBalancedRec(rest, numOpen);
}

And call this like IsBalancedRec("so(m(eth)ing)").