Surrounded Regions

Q:

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 XX O O XX X O XX O X X

After running your function, the board should be:

X X X XX X X XX X X XX O X X

Solution:

Find all the connected 'O' cells from 4 edges and mark them with 'Y', those cells should not be filled. Then fill all the remaining 'O' cells with 'X' and recover all the 'Y' cells.

public class Solution {    public void solve(char[][] board) {        if (board.length == 0) return;        int m = board.length;        if (board[0].length == 0) return;        int n = board[0].length;        for (int i = 0; i < m; i++) {            fill(board, i, 0);            fill(board, i, n-1);        }        for (int i = 1; i < n-1; i++) {            fill(board, 0, i);            fill(board, m-1, i);        }        for (int i = 0; i < m; i++) {            for (int j = 0; j < n; j++) {                if (board[i][j] == 'O')                    board[i][j] = 'X';                else if (board[i][j] == 'Y')                    board[i][j] = 'O';            }        }    }        void fill(char[][] board, int i, int j) {        if (board[i][j] != 'O') return;        board[i][j] = 'Y';        Queue<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 - row * board[0].length;            if (row > 1 && board[row-1][col] == 'O') {                queue.offer((row-1)*board[0].length + col);                board[row-1][col] = 'Y';            }            if (row < board.length-2 && board[row+1][col] == 'O') {                queue.offer((row+1)*board[0].length + col);                board[row+1][col] = 'Y';            }            if (col > 1 && board[row][col-1] == 'O') {                queue.offer(row*board[0].length + col - 1);                board[row][col-1] = 'Y';            }            if (col < board[0].length-2 && board[row][col+1] == 'O') {                queue.offer(row*board[0].length + col + 1);                board[row][col+1] = 'Y';            }        }    }}