code: public Set<Point> diff(Maze2D to) {
          char[][] maze1 = this.getMazeCharArray();
          char[][] maze2 = to.getMazeCharArray();
          Set<Point> differentLocations = new HashSet<Point>();
          for (int row = 0; row < this.rows; row++) {
              for (int column = 0; column < this.columns; column++) {
                  if (maze1[row][column] != maze2[row][column]) {
                      differentLocations.add(new Point(column, row));
                  }
              }
          }
          return differentLocations;
      }
docs: Returns a set of points that are different with respect this maze.
  Both mazes must have same size.
  @param to maze to be compared.
  @return set of different points.