Why my code fails for the testcase discussed below? (Leetcode problem number 130)

Testcase:
Input
[["X","O","X"],["X","O","X"],["X","O","X"]]
My Output:
[["X","O","X"],["O","O","O"],["X","O","X"]]
Expected Output:
[["X","O","X"],["X","O","X"],["X","O","X"]]


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