1 class Solution {
 2 public:
 3     void solve(vector<vector<char>>& board) {
 4         if (board.size() == 0) return;
 5         rows = board.size();
 6         cols = board[0].size();
 7         for (int i = 0; i < rows; i++) {
 8             for (int j = 0; j < cols; j++) {
 9                 if (i > 0 && i < rows - 1 && j > 0 && j < cols - 1 && board[i][j] == 'O') {
10                     searchAndRecover(board, i, j);
11                 }
12             }
13         }
14         for (int i = 0; i < rows; i++) {
15             for (int j = 0; j < cols; j++) {
16                 if (board[i][j] == 'F') {
17                     board[i][j] = 'O';
18                 }
19             }
20         }
21     }
22     void searchAndRecover(vector<vector<char>>& board, int x, int y) {
23         // intialize
24         searching.clear();
25         recover.clear();
26         int directions[4][2] = {{1, 0}, {0, -1}, {-1, 0}, {0, 1}};
27         bool need_recovery = false;
28         // store the original one
29         pair<int, int> origin(x, y);
30         searching.push_back(origin);
31         while (!searching.empty()) {
32             // get the top of the stack
33             pair<int, int> tmp = searching.back();
34             searching.pop_back();
35             // change the value
36             board[tmp.first][tmp.second] = 'X';
37             recover.push_back(tmp);
38             for (int i = 0; i < 4; i++) {
39                 int tmp_x = tmp.first + directions[i][0];
40                 int tmp_y = tmp.second + directions[i][1];
41                 if (tmp_x >= 0 && tmp_x < rows && tmp_y >= 0 && tmp_y < cols && board[tmp_x][tmp_y] == 'O') {
42                     if (tmp_x > 0 && tmp_x < rows - 1 && tmp_y > 0 && tmp_y < cols - 1) {
43                         pair<int, int> add_point(tmp_x, tmp_y);
44                         searching.push_back(add_point);
45                     }
46                     else {
47                         need_recovery = true;
48                     }
49                 }
50             }
51         }
52         if (need_recovery) {
53             while (!recover.empty()) {
54                 pair<int, int> tmp = recover.back();
55                 board[tmp.first][tmp.second] = 'F';
56                 recover.pop_back();
57             }
58         }
59     }
60 private:
61     int rows;
62     int cols;
63     vector<pair<int, int>> searching;
64     vector<pair<int, int>> recover;
65 };