public static int alternate(string s)
    {
        int maxLength = 0;
        HashSet<char> uset = new HashSet<char>(s.ToCharArray());
        char[] chars = uset.ToArray();
        for (int i = 0; i < chars.Length; i++)
            for (int j = i+1; j < chars.Length; j++) {
                int len = GetDuoCharsLength(s, chars[i], chars[j]);
                if (len > maxLength) maxLength = len;
            }
            
        return maxLength;
    }
    
    private static int GetDuoCharsLength(string s, char ch1, char ch2) {
        if (s.IndexOf(ch1) > s.IndexOf(ch2)) {
            char temp = ch1;
            ch1 = ch2;
            ch2 = temp;
        }
        int len = 0;
        bool turn = true;
        foreach (char ch in s) {
            if (ch != ch1 && ch != ch2) continue;
            if ((turn && ch == ch1) || (!turn && ch == ch2)) {
                turn = !turn;
                len++;
            } else {
                return 0;
            }
        }
        return len;
        
    }