Surrounded Regions

Given a 2D board containing 'X' and 'O', capture all regions surrounded by 'X'.

A region is captured by flipping all 'O's into 'X's in that surrounded region.

For example,

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

 

After running your function, the board should be:

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

public class Solution {
    public void solve(char[][] board) {
		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';
			}
		}
    }

	private void fill(char[][] board, int i, int j) {
		if (board[i][j]!='O') {
			return;
		}
		board[i][j] = '#';
		Queue<Integer> queue = new LinkedList<Integer>();
		int code = i*board[0].length+j;
		queue.add(code);
		while (!queue.isEmpty()) {
			code = queue.poll();
			int row = code/board[0].length;
			int col = code%board[0].length;
			if (row >= 1 && board[row - 1][col] == 'O') {
				queue.add((row - 1) * board[0].length + col);
				board[row - 1][col] = '#';
			}

			if (row <= board.length - 2 && board[row + 1][col] == 'O') {
				queue.add((row + 1) * board[0].length + col);
				board[row + 1][col] = '#';
			}

			if (col >= 1 && board[row][col - 1] == 'O') {
				queue.add(row * board[0].length + col - 1);
				board[row][col - 1] = '#';
			}

			if (col <= board[0].length - 2 && board[row][col + 1] == 'O') {
				queue.add(row * board[0].length + col + 1);
				board[row][col + 1] = '#';
			}
		}
	}
}