Solution.cpp

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        int m = board.size(), n = board[0].size();
        int dirs[5] = {-1, 0, 1, 0, -1};
        function<void(int, int)> dfs = [&](int i, int j) {
            if (i < 0 || i >= m || j < 0 || j >= n || board[i][j] != 'O') {
                return;
            }
            board[i][j] = '.';
            for (int k = 0; k < 4; ++k) {
                dfs(i + dirs[k], j + dirs[k + 1]);
            }
        };
        for (int i = 0; i < m; ++i) {
            dfs(i, 0);
            dfs(i, n - 1);
        }
        for (int j = 1; j < n - 1; ++j) {
            dfs(0, j);
            dfs(m - 1, j);
        }
        for (auto& row : board) {
            for (auto& c : row) {
                if (c == '.') {
                    c = 'O';
                } else if (c == 'O') {
                    c = 'X';
                }
            }
        }
    }
};