Sort Characters by Frequency Using JAVA 8

class SortCharactersByFrequency {

    /**

     * Java Streams Approach

     * Time complexity : O(N)

     * Space complexity : O(N)

     */

    static String frequencySort(String s) {

        return s.chars()

                .mapToObj(Character::toString)                        //map to string character

                .collect(Collectors.groupingBy(Function.identity()))  //collect to Map<String, List<String>>

                .values()                                             //drop keys, we care only about values

                .stream()

                .sorted(Comparator.<List<String>>comparingInt(List::size).reversed()) //sort by list size in descending order

                .flatMap(strings -> strings.stream())                                 //flat map to stream of character strings

                .collect(Collectors.joining());                                       //build a string from the stream

    }

}