Surrounded Regions

public class Solution {
    public void solve(char[][] board) {
        if(board==null || board.length==0 || board[0].length==0)    return;
        int row = board.length;
        int col = board[0].length;
        
        for(int i=0; i<row; i++)
        {
            fill(board, i, 0);
            fill(board, i, col-1);
        }
        for(int j=0; j<col; j++)
        {
            fill(board, 0, j);
            fill(board, row-1, j);
        }
        
        for(int i=0; i<row; i++)
        {
            for(int j=0; j<col; j++)
            {
                if(board[i][j] == 'O')
                {
                    board[i][j] = 'X';
                }
                else if(board[i][j] == '#')
                {
                    board[i][j] = 'O';
                }
                
            }
        }
    }
    public void fill(char[][] board, int i, int j)
    {
        int row = board.length;
        int col = board[0].length;
        if(board[i][j] != 'O') return;
        board[i][j] = '#';
        LinkedList<Integer> queue = new LinkedList<Integer>();
        queue.offer(i*col + j);
        while(!queue.isEmpty())
        {
            int code = queue.poll();
            int r = code/col;
            int c = code%col;
            if(r>0 && board[r-1][c]=='O')
            {
                queue.offer((r-1)*col+c);
                board[r-1][c] = '#';
            }
            if(r<row-1 && board[r+1][c]=='O')
            {
                queue.offer((r+1)*col+c);
                board[r+1][c] = '#';
            }
            
            if(c<col-1 && board[r][c+1]=='O')
            {
                queue.offer(r*col+(c+1));
                board[r][c+1] = '#';
            }
            if(c>0 && board[r][c-1]=='O')
            {
                queue.offer(r*col+(c-1));
                board[r][c-1] = '#';
            }
        }
    }
}