public static int alternate(String str) {
    String[] uniqueCharacters = Arrays.stream(str.split("")).collect(Collectors.toSet()).toArray(new String[0]);
    int maxLength = 0;
    for(int i=0;i<uniqueCharacters.length-1; i++) {
        for(int j=i+1; j<uniqueCharacters.length; j++) {
            String firstChar = uniqueCharacters[i];
            String secondChar= uniqueCharacters[j];
            String newStr = str;
            List<String> lst = Arrays.stream(uniqueCharacters).filter(s -> !s.equals(firstChar) && !s.equals(secondChar)).collect(Collectors.toList());
            for(String l:lst) {
                newStr = newStr.replaceAll(l, "");
            }
            if(isAlternateCharacters(newStr) & newStr.length() > maxLength) {
                maxLength = newStr.length();
            }
        }
    }
    return maxLength;
}   


private static boolean isAlternateCharacters(String str) {
    if(str.length() == 1) return false;

    char firstchar = str.charAt(0);
    char secondchar = str.charAt(1);
    if(firstchar == secondchar) return false;

    for(int i=0; i<str.length(); i++) {
        if(i%2 == 0 && str.charAt(i) == firstchar) continue;
        if(i%2 != 0 && str.charAt(i) == secondchar) continue;
        return false;
    }
    return true;
}