//TODO
            // Marks the cell [i, j] as visited and add its non visited neighbour to the frontier
            marked[i, j] = true;
            int l = marked.GetLength(0);
            if ( i < l - 1 && !marked[i + 1,j] && !Exists(frontier, i + 1, j))
            {
                frontier.Add((i + 1,j));
            }
            if ( j > 0 && !marked[i,j - 1] && !Exists(frontier, i, j - 1 ))
            {
                frontier.Add((i, j - 1 ));
            }
            if ( i > 0 && !marked[i - 1,j] && !Exists(frontier, i - 1, j))
            {
                frontier.Add((i - 1 ,j));
            }
            if (j < l - 1 && !marked[i,j+1] && !Exists(frontier, i, j + 1))
            {
                frontier.Add((i, j + 1));
            }