public class testerei {

    public static void main(String[] args) {

        System.out.println(recStr("acbxdyfjzdmk"));

    }

    private static String recStr(String s){
        if(s.length() <= 0)
            return "";

        int maxval = s.charAt(0);

        int nextSubstringMaxval = -1;
        String nextSubstring = recStr(s.substring(1));
        if(nextSubstring.length() > 0)
            nextSubstringMaxval = nextSubstring.charAt(0);


        if(maxval > nextSubstringMaxval)
            return (char)maxval+s.replace(""+maxval, "");
        else
            return (char)nextSubstringMaxval+s.replace(""+nextSubstringMaxval, "");


    }

}