class Grid{
  public final int size;

  Cell[] cells;
  Cell[][] rows;
  Cell[][] cols;
  Cell[][] squares;

  // Constructs a 9x9 grid
  public Grid(){
    this(9);
  }

  // Constructs a size x size grid
  public Grid(int size){
    this.size = size;
    cells = new Cell[size * size];
    
    // Allocate subsets
    rows = new Cell[size][size];
    cols = new Cell[size][size];
    squares = new Cell[size][size];

    // Initialize the cells
    for(int i=0; i<cells.length; i++){
      cells[i] = new Cell(size);
    }

    // Build the row subsets
    for(int i=0; i<size; i++)
      System.arrayCopy(cells, size*i, rows[i], 0, size);

    // Build the column subsets
    for(int i=0; i<size; i++){
      for(int j=0; j<size; j++)
        col[i][j] = cells[i + size*j];
    }

    // Build the square subsets
    int len = (int)Math.sqrt(size);
    for(int i=0; i<cells.length; i+=size){
      System.arrayCopy(cells, i, squares[i], 0, len);
      System.arrayCopy(cells, i+size, squares[i], len, len);
      System.arrayCopy(cells, i+2*size, squares[i], 2*len, len);
    }
  }

  // Rest of class snipped
}