1 class Solution {
 2 public:
 3     const int dx[5] = {0, 0, 1, -1};
 4     const int dy[5] = {1, -1, 0, 0};
 5     int n, m;
 6     bool ok(int x, int y) {
 7         return x >= 0 && x < n && y >= 0 && y < m;
 8     }
 9     void dfs(vector<vector<char>>& board, int x, int y) {
10         board[x][y] = ‘Q‘;
11         for(int i = 0; i < 4; i++) {
12             int xx = x + dx[i];
13             int yy = y + dy[i];
14             if(ok(xx, yy) && board[xx][yy] == ‘O‘) {
15                 dfs(board, xx, yy);
16             }
17         }
18     }
19     void solve(vector<vector<char>>& board) {
20         n = board.size();
21         if(n == 0) return;
22         m = board[0].size();
23         for(int i = 0; i < n; i++) {
24             if(board[i][0] == ‘O‘) dfs(board, i, 0);
25             if(board[i][m-1] == ‘O‘) dfs(board, i, m-1);
26         }
27         for(int i = 0; i < m; i++) {
28             if(board[0][i] == ‘O‘) dfs(board, 0, i);
29             if(board[n-1][i] == ‘O‘) dfs(board, n-1, i);            
30         }
31         for(int i = 0; i < n; i++) {
32             for(int j = 0; j < m; j++) {
33                 if(board[i][j] == ‘O‘) board[i][j] = ‘X‘;
34                 else if(board[i][j] == ‘Q‘) board[i][j] = ‘O‘;
35             }
36         }
37     }
38 };