130 被围绕的区域

https://leetcode.cn/leetbook/read/top-interview-questions-hard/xwfor1/

cost: 28m07s

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        for (int i = 0; i < board.size(); i++) {
            this->dfs(board, i, 0);
            this->dfs(board, i, board[0].size() - 1);
        }
        for (int i = 0; i < board[0].size(); i++) {
            this->dfs(board, 0, i);
            this->dfs(board, board.size() - 1, i);
        }
        for (int i = 0; i < board.size(); i++) {
            for (int j = 0; j < board[0].size(); j++) {
                char& ch = board[i][j];
                if (ch == '1') {
                    ch = 'O';
                } else if (ch == 'O') {
                    ch = 'X';
                }
            }
        }
    }

private:
    void dfs(vector<vector<char>>& board, int i, int j) {
        if (i < 0 || j < 0 || i >= board.size() || j >= board[0].size() ||
            board[i][j] != 'O') {
            return;
        }
        board[i][j] = '1';
        this->dfs(board, i - 1, j);
        this->dfs(board, i + 1, j);
        this->dfs(board, i, j - 1);
        this->dfs(board, i, j + 1);
    }
};