private void getAllFeasibleCharacters(char output[], int k,int pos,Set<Character> allChars){
        for(int i = pos-1; i > pos -k && i >=0; i--){
            allChars.remove(output[i]);
        }
    }
http://www.geeksforgeeks.org/rearrange-a-string-so-that-all-same-characters-become-at-least-d-distance-away/
src/com/interview/string/RearrangeDuplicateCharsdDistanceAway.java