public static String getLetters(String s){
    lettersCount = new int[26];
    skippable = new int[26];
    for(int j=0; j<s.length(); j++){
        int index = s.charAt(j)-97;
        lettersCount[index]+=1;
    }
    for(int i=0; i<skippable.length; i++){
        lettersCount[i]/=2;
        skippable[i] = lettersCount[i];
    }
    buffer = new ArrayList<Integer>();
    String finalString = "";
    for(int i=s.length()-1; i>=0; i--){
        char currChar = s.charAt(i);
        if(buffer.size()==0)
            buildBuffer(i);

        boolean isSmallest = true;
        for(Integer l:buffer){
            if(currChar>l){
                isSmallest=false;
            }
        }
        if(isSmallest&&lettersCount[(int)(currChar-97)]>0){
            finalString += currChar;
            lettersCount[(int)(currChar-97)]--;
        }
        buffer.remove(0);
    }
    return finalString;
}

public static void buildBuffer(int index){
    for(int i=index; i>=0; i--){
        if(skippable[s.charAt(i)-97]>=0){
            buffer.add((int)s.charAt(i));
            if(skippable[s.charAt(i)-97]==0)
                return;
            skippable[(int)(s.charAt(i)-97)]--;
            //System.out.println(skippable[s.charAt(i)-97]);
        }
        else
            return;
    }
    return;
}

Whats the problem with this code? It works for sometest cases