my code is as follow, the main idea is dfs the four boundary and set 'O' flag to 'v', then change 'O' to 'X' and 'v' to 'O'.

But I got the Runtime Error while the output is the same as the expected output?

Any one can help me to point out where is the error?

```
class Solution {
void dfs(vector<vector<char>>& board, int i, int j) {
int m = board.size(), n = board[0].size();
if (i < 0 || i >= m || j < 0 || j >= n || board[i][j] == 'X' || board[i][j] == 'v')
return;
board[i][j] = 'v';
dfs(board, i + 1, j);
dfs(board, i - 1, j);
dfs(board, i, j + 1);
dfs(board, i, j - 1);
}
public:
void solve(vector<vector<char>>& board) {
if (board.size() == 0 || board[0].size() == 0)
return;
int m = board.size(), n = board[0].size();
for (int i = 0; i < m; i++) {
dfs(board, i, 0);
dfs(board, i, n-1);
}
for (int j = 0; j < n; j++) {
dfs(board, 0, j);
dfs(board, m-1, j);
}
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] == 'v')
board[i][j] = 'O';
}
}
}
};
```