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


从四个边入手，把不能换成X的O标记出来，用map存。 然后再遍历这个array， 更改可以变成X的。

public class Solution {
    public void solve(char[][] board) {
        // Start typing your Java solution below
        // DO NOT write main() function
        if(board == null || board.length == 0 || board[0].length == 0) return;
        boolean[][] map = new boolean[board.length][board[0].length];
        int len = board[0].length;
        LinkedList<Integer> ll = new LinkedList<Integer>();
        for(int i = 0; i < board.length; i ++){
            if(board[i][0] == 'O' && !map[i][0]){
                map[i][0] = true;
                ll.push(i * len);
            } 
            if(board[i][len - 1] == 'O' && !map[i][len - 1]){ 
                map[i][len - 1] = true;
                ll.push(i * len + len - 1);
            }
        }
        for(int i = 0; i < len; i ++){
            if(board[0][i] == 'O' && !map[0][i]){
                map[0][i] = true;
                ll.push(i);
            } 
            if(board[board.length - 1][i] == 'O' && !map[board.length - 1][i]){ 
                map[board.length - 1][i] = true;
                ll.push((board.length - 1) * len + i);
            }
        }
        while(ll.size() != 0){
            int tmp = ll.poll();
            int i = tmp / len;
            int j = tmp % len;
            if(i > 1 && board[i - 1][j] == 'O' && !map[i - 1][j]){
                map[i - 1][j] = true;
                ll.push((i - 1) * len + j);
            }
            if(j > 1 && board[i][j - 1] == 'O' && !map[i][j - 1]){
                map[i][j - 1] = true;
                ll.push(i * len + j - 1);
            }
            if(i + 1 < board.length && board[i + 1][j] == 'O' && !map[i + 1][j]){
                map[i + 1][j] = true;
                ll.push((i + 1) * len + j);
            }
            if(j + 1 < len && board[i][j + 1] == 'O' && !map[i][j + 1]){
                map[i][j + 1] = true;
                ll.push(i * len + j + 1);
            }
        }
        for(int i = 0; i < board.length; i ++)
            for(int j = 0; j < len; j ++)
                if(!map[i][j] && board[i][j] == 'O')
                    board[i][j] = 'X';
    }
}