class Solution {
public:
    bool isValid(int i, int j, vector<vector<char>>& board) {
        return i >= 0 and i < board.size() and j >= 0 and j < board[0].size() and board[i][j] == 'O';
    }
    bool isBorder(int i, int j, vector<vector<char>>& board) {
        return i == 0 or j == 0 or i == board.size() - 1 or j == board[0].size() - 1;
    }
    void dfs(int i, int j, vector<vector<char>>& board, set<pair<int, int>>& visited, set<pair<int, int>>& curr_visited) {
        if(!isValid(i, j, board) or visited.find({i, j}) != visited.end()) return;
        if(isBorder(i, j, board)) {
            for(auto element : curr_visited) {
                int x = element.first;
                int y = element.second;
                board[x][y] = 'O';
            }
            return;
        }
        visited.insert({i, j});
        curr_visited.insert({i, j});
        board[i][j] = 'X';
        vector<pair<int, int>> dirs = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
        for(auto dir : dirs) {
            int dx = i + dir.first;
            int dy = j + dir.second;
            dfs(dx, dy, board, visited, curr_visited);
        }
    }
    void solve(vector<vector<char>>& board) {
        set<pair<int, int>> visited;
        for(int i = 0; i < board.size(); i++) {
            for(int j = 0; j < board[0].size(); j++) {
                if(board[i][j] == 'O') {
                    set<pair<int, int>> curr_visited;
                    dfs(i, j, board, visited, curr_visited);
                }
            }
        }
    }
};