Solution.java

/***
* @param board ChessBoard
* @param pieces Chess Pieces selected by the user
* @param solutions valid solutions to the problem
* @param testedConfigurations Board configurations to which solutions were not found
* @return return a list of possible solutions to the problem in the form of a list of filled chess boards
*/
public static Set<Board> solution(Board board, List<Character> pieces, Set<Board> solutions, Set<Board> testedConfigurations) {
  if (!pieces.isEmpty()) { // if pieces is empty I return the solutions
      for (int i = 1; i < board.getM(); i++) {
        for (int j = 1; j < board.getN(); j++) {
          // I iterate over the dimensions of the board, create a new piece, try to place it on
          // the board. If it's safe I add it to the used pieces and recursively call
          // the solution method. I have an auxiliary collection, so I don't try board configurations twice
          ChessPiece c = PieceFactory.createPiece(pieces.get(0), i, j);
          if (board.isSafe(c)) {
            Board b = board.place(c); 
            if (pieces.size() != 1) {
              if (testedConfigurations.add(b)) {
                solution(b, ImmutableUtils.removeFirstPiece(pieces), solutions, testedConfigurations);
              }
            } else {
              solutions.add(b);
            }
        }
      }
    }
  }
  return solutions;
}