/**
* opens an element in grid
*
* @param i - row
* @param j - column
*/
public void open(final int i, final int j) {
checkBoundary(i, j);
if (!isOpen(i, j)) {
grid[i][j] = true;
}
connectVirtualTop(i, j);
connectVirtualBottom(i, j);
connectLeftNeighbour(i, j);
connectRightNeighbour(i, j);
connectTopNeighbour(i, j);
connectBottomNeighbour(i, j);
}
