130. 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

 

 

 

Similar: 

• 286. Walls and Gates
• 200. Number of Islands

public class Solution {
    public void solve(char[][] board) {
        if (board.length == 0 || board[0].length == 0)
            return;
        int m = board.length;
        int n = board[0].length;
        
        Queue<int[]> queue = new LinkedList<int[]>();
        for (int i = 0; i < m; i++) {
            if (board[i][0] == 'O') {
                board[i][0] = 'H';
                queue.offer(new int[] {i,0});
            }
            if (board[i][n-1] == 'O') {
                board[i][n-1] = 'H';
                queue.offer(new int[] {i,n-1});
            }
        }
        for (int j = 1; j < n-1; j++) {
            if (board[0][j] == 'O') {
                board[0][j] = 'H';
                queue.offer(new int[] {0,j});
            }
            if (board[m-1][j] == 'O') {
                board[m-1][j] = 'H';
                queue.offer(new int[] {m-1,j});
            }
        }
        
        int[] shift = {1, 0, -1, 0, 1};
        while (!queue.isEmpty()) {
            int[] pt = queue.poll();
            for (int i=0; i<4; i++) {
                int r = pt[0] + shift[i];
                int c = pt[1] + shift[i+1];
                if (r>=0 && r<m && c>=0 && c<n && board[r][c]=='O') {
                    board[r][c] = 'H';
                    queue.offer( new int[] {r,c});
                }
            }
        }
        
        for (int i=0; i<m; i++)
            for (int j=0; j<n ;j++) {
                if (board[i][j] =='O') {
                    board[i][j] = 'X';
                }
            }
        for (int i=0; i<m; i++)
            for (int j=0; j<n; j++) {
                if (board[i][j] == 'H') {
                    board[i][j] = 'O';
                }
            }
    }
}