public void backtrackSolve(MutableBoard grid) {
BoardInfoHelper helper = new BoardInfoHelper(grid);
List<Position> positions = helper.getUnknownBorderCells();
toPick = new HashMap<>(); toFlag = new HashMap<>();
toPickSet = new HashSet<>(positions); toFlagSet = new HashSet<>(positions);
backtrackSolve(grid, helper, positions, new Node(null,null), new Node(null,null));
System.out.println("To Pick " + toPickSet);
System.out.println("To Flag " + toFlagSet);
for(Position p : toPickSet)
grid.setCell(p, Cell.NO_BOMB);
for(Position p : toFlagSet)