C#

public static int alternate(string s) { // Step 1: Get all unique characters in the string HashSet uniqueChars = new HashSet(s);

    int maxLength = 0;

    // Step 2: Try every pair of unique characters
    foreach (char char1 in uniqueChars)
    {
        foreach (char char2 in uniqueChars)
        {
            if (char1 != char2)
            {
                int currentLength = 0;
                char lastChar = '\0';
                bool isValid = true;

                // Step 3: Loop through the string
                foreach (char c in s)
                {
                    if (c == char1 || c == char2)
                    {
                        // If two consecutive characters are the same, it's invalid
                        if (c == lastChar)
                        {
                            isValid = false;
                            break;
                        }
                        lastChar = c;
                        currentLength++;
                    }
                }

                // If the string is valid, update maxLength
                if (isValid)
                {
                    maxLength = Math.Max(maxLength, currentLength);
                }
            }
        }
    }

    return maxLength;
}