JAVA add code in ???

 determine the correct order of piece placements 

 Given a string describing a placement of pieces and a string describing  * an (unordered) objective, return a set of all possible next viable  * piece placements. A viable piece placement must be a piece that is  * not already placed (ie not in the placement string), and which will not  * obstruct any other unplaced piece.  *  * @param placement A valid sequence of piece placements where each piece placement is drawn from the objective  * @param objective A valid game objective, but not necessarily a valid placement string  * @return An set of viable piece placements  */ static Set getViablePiecePlacements(String placement, String objective) {   return null; }