0130.cpp

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        if (board.empty()) return;

        const int m = board.size();
        const int n = board[0].size();

        for (int i = 0; i < m; i++) {
            dfs(board, i, 0);
            dfs(board, i, n - 1);
        }

        for (int j = 1; j < n - 1; j++) {
            dfs(board, 0, j);
            dfs(board, m - 1, j);
        }

        for (int i = 0; i < m; i++)
            for (int j = 0; j < n; j++)
                board[i][j] = board[i][j] == '.' ? 'O' : '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] = '.';

        dfs(board, i, j + 1);
        dfs(board, i, j - 1);
        dfs(board, i + 1, j);
        dfs(board, i - 1, j);
    }
};