// List to store all permutations
    List<string> permutations = new List<string>();
    
    public List<string> findLetterCaseStringPermutations(string str)
    {
        // Begin the process
        find(currPermutation: str.ToCharArray(), index: 0);
        return permutations;
    }
    
    private void find(char[] currPermutation, int index)
    {
        // Base case: if index is out of bounds, add the current permutation to the list
        if (index == currPermutation.Length)
        {
            permutations.Add(string.Concat(currPermutation));
            return;
        }

        // If character is letter, toggle case and find next permutation 
        if (char.IsLetter(currPermutation[index]))
        {
            // Toggle case for the character at the current index
            currPermutation[index] = toggleCase(currPermutation[index]);
            // Recursively find the next permutation: with toggled case
            find(currPermutation, index + 1);
            // Backtrack: undo the toggle case for the character at the current index
            currPermutation[index] = toggleCase(currPermutation[index]);
        }
        // Recursively find the next permutation
        // For letter it will be untoggled
        find(currPermutation, index + 1);
    }

    private char toggleCase(char c)
    {
        return char.IsUpper(c) ? char.ToLower(c) : char.ToUpper(c);
    }