Surrounded Regions — C++

// Time:  O(m * n)
// Space: O(m + n)

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

        queue<pair<int, int>> q;
        for (int i = 0; i < board.size(); ++i) {
            if (board[i][0] == 'O') {
                board[i][0] = 'V';
                q.emplace(i, 0);
            }
            if (board[i][board[0].size() - 1] == 'O') {
                board[i][board[0].size() - 1] = 'V';
                q.emplace(i, board[0].size() - 1);
            }
        }

        for (int j = 1; j < board[0].size() - 1; ++j) {
            if (board[0][j] == 'O') {
                board[0][j] = 'V';
                q.emplace(0, j);
            }
            if (board[board.size() - 1][j] == 'O') {
                board[board.size() - 1][j] = 'V';
                q.emplace(board.size() - 1, j);
            }
        }
        while (!q.empty()) {
            int i, j;
            tie(i, j) = q.front();
            q.pop();
            static const vector<pair<int, int>> directions{{0, -1}, {0, 1},
                                                           {-1, 0}, {1, 0}};
            for (const auto& d : directions) {
                const int x = i + d.first, y = j + d.second;
                if (0 <= x && x < board.size() &&
                    0 <= y && y < board[0].size() &&
                    board[x][y] == 'O') {
                    board[x][y] = 'V';
                    q.emplace(x, y);
                }
            }
        }
        for (int i = 0; i < board.size(); ++i) {
            for (int j = 0; j < board[0].size(); ++j) {
                if (board[i][j] != 'V') {
                    board[i][j] = 'X';
                } else {
                    board[i][j] = 'O';
                }
            }
        }
    }
};