void dfs(vector<string>& board,int i, int j)

{

if (i < 0 || i >= board.size()) return;

if (j < 0 || j >= board[0].size()) return;

```
if (board[i][j] == 'X'||board[i][j] == '$') return;
board[i][j] = '$';
dfs(board, i - 1, j);
dfs(board, i + 1, j);
dfs(board, i, j - 1);
dfs(board, i, j + 1);
```

}

void solve(vector<string>& board)

{

if (board.empty() || board[0].empty()) return;

int m = board.size(), n = board[0].size();

```
for (int i = 0; i < m; i++)
{
for (int j = 0; j < n; j++)
{
if (i==0||j==0||i==m-1||j==n-1)
{
if (board[i][j] == 'O') dfs(board, i, j);
}
}
}
cout << board[0][0] << endl;
for (int i = 0; i < m; i++)
{
for (int j = 0; j < n; j++)
{
if (board[i][j] == 'O') board[i][j] = 'X';
else if (board[i][j] == '$') board[i][j] = 'O';
}
}
```

}