Surrounded Regions

1.题目

给一个二维的矩阵，包含 'X' 和 'O', 找到所有被 'X' 围绕的区域，并用 'X' 填充满

给出二维矩阵：

X X X X
X O O X
X X O X
X O X X

把被 'X' 围绕的区域填充之后变为：

X X X X
X X X X
X X X X
X O X X

2.算法

这道题分两部走，第一步是用Flood fill算法从矩阵四周向内遍历，找到没有被 'X' 围绕的O，并把他们赋值为#，第二步是，被我们换成'#'的那些顶点，剩下的所有'O'都应该被替换成'X'，而'#'那些最终应该是还原成'O'。

Flood fill算法的广度实现方法能把四周相同的O都变为#

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