struct Coord {
    int x;
    int y;
    Coord(int tmp_x, int tmp_y): x(tmp_x), y(tmp_y) {}
};

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        if (board.empty() || board.size() == 0 || board[0].empty() || board[0].size() == 0) return;
        four_ways = {{0, 1}, {0, -1}, {1, 0}, {-1, 0}};
        for (int i = 0; i < board.size(); i++) {
            for (int j = 0; j < board[0].size(); j++) {
                if (i == 0 || i == board.size() - 1 || j == 0 || j == board[0].size() - 1) {
                    if (board[i][j] == 'O') {
                        Coord c(i, j);
                        bfs(board, c);
                    }
                }
            }
        }
        for (int i = 0; i < board.size(); i++) {
            for (int j = 0; j < board[0].size(); j++) {
                if (board[i][j] == 'O') board[i][j] = 'X';
                else if (board[i][j] == 'Y') board[i][j] = 'O';
            }
        }
    }
private:
    vector<vector<int>> four_ways;
    void bfs(vector<vector<char>>& board, Coord& c) {
        queue<Coord> Q;
        Q.push(c);
        board[c.x][c.y] = 'Y';
        while(!Q.empty()) {
            Coord tmp_c = Q.front(); Q.pop();
            for (int i = 0; i < 4; i++) {
                Coord new_c(tmp_c.x + four_ways[i][0], tmp_c.y + four_ways[i][1]);
                if (is_valid(board, new_c)) {
                    Q.push(new_c);
                    board[new_c.x][new_c.y] = 'Y';
                }
            }
        }
    }
    bool is_valid(vector<vector<char>>& board, Coord& c) {
        if (c.x < 0 || c.x >= board.size() || c.y < 0 || c.y >= board[0].size() || board[c.x][c.y] == 'X' || board[c.x][c.y] == 'Y') return false;
        return true;
    }
};