leetcode-----130. 被围绕的区域

• 链接：https://leetcode-cn.com/problems/surrounded-regions/

代码

/*
 * @lc app=leetcode.cn id=130 lang=cpp
 *
 * [130] 被围绕的区域
 * flood fill
 */

// @lc code=start
class Solution {
public:
    int n, m;
    vector<vector<char>> board;
    int dx[4] = {-1, 0, 1, 0}, dy[4] = {0, 1, 0, -1};

    void solve(vector<vector<char>>& _board) {
        board = _board;
        n = board.size();
        if (!n) return;
        m = board[0].size();

        for (int i = 0; i < n; i ++ ) {
            if (board[i][0] == 'O') dfs(i, 0);
            if (board[i][m - 1] == 'O') dfs(i, m - 1);
        }

        for (int i = 0; i < m; i ++ ) {
            if (board[0][i] == 'O') dfs(0, i);
            if (board[n - 1][i] == 'O') dfs(n - 1, i);
        }

        for (int i = 0; i < n; i ++ )
            for (int j = 0; j < m; j ++ )
                if (board[i][j] == '#') board[i][j] = 'O';
                else board[i][j] = 'X';

        _board = board;
    }

    void dfs(int x, int y) {
        board[x][y] = '#';
        for (int i = 0; i < 4; i ++ ) {
            int a = x + dx[i], b = y + dy[i];
            if (a >= 0 && a < n && b >= 0 && b < m && board[a][b] == 'O')
                dfs(a, b);
        }
    }
};
// @lc code=end