130. 被围绕的区域

//1、从四条边出发，找到与"O"相连的，并把当前"O"置为"#"
//2、把"O"改为"X"，把"#"改为"O"
class Solution 
{
public:
    void solve(vector<vector<char>>& board) 
    {
        if (board.size() == 0 || board.size() == 0) return;
        int m = board.size();
        int n = board[0].size();
        for (int i = 0; i < m; i++) 
        {
            for (int j = 0; j < n; j++) 
            {
                // 从边缘'O'开始搜索
                bool isEdge = i == 0 || j == 0 || i == m - 1 || j == n - 1;
                if (isEdge && board[i][j] == 'O')
                {
                    dfs(board, i, j);
                }
            }
        }

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

    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] == 'X' || board[i][j] == '#') 
        {
            // board[i][j] == '#' 说明已经搜索过了. 
            return;
        }
        board[i][j] = '#';
        dfs(board, i - 1, j); // 上
        dfs(board, i + 1, j); // 下
        dfs(board, i, j - 1); // 左
        dfs(board, i, j + 1); // 右
    }
};