Solution2.cpp

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        int m = board.size(), n = board[0].size();
        vector<int> p(m * n + 1);
        iota(p.begin(), p.end(), 0);
        function<int(int)> find = [&](int x) {
            return p[x] == x ? x : p[x] = find(p[x]);
        };
        int dirs[5] = {-1, 0, 1, 0, -1};
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (board[i][j] == 'O') {
                    if (i == 0 || i == m - 1 || j == 0 || j == n - 1) {
                        p[find(i * n + j)] = find(m * n);
                    } else {
                        for (int k = 0; k < 4; ++k) {
                            int x = i + dirs[k];
                            int y = j + dirs[k + 1];
                            if (board[x][y] == 'O') {
                                p[find(x * n + y)] = find(i * n + j);
                            }
                        }
                    }
                }
            }
        }
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (board[i][j] == 'O' && find(i * n + j) != find(m * n)) {
                    board[i][j] = 'X';
                }
            }
        }
    }
};