思想很重要

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        int m = board.size();
        int n = board[0].size();
        // 從邊緣開始，因為四周肯定不被包圍，和四周O聯通的先呼叫dfs演算法標記為A
        for(int i = 0; i < m; i++){
            dfs(board,i,0);
            dfs(board,i,n-1);
        }
        
 
for(int i = 0; i < n; i++){
            dfs(board,0,i);
            dfs(board,m-1,i);
        }
        // 最後剩下的肯定O肯定是被X包圍的
        // 最後只需要將標記為A的復原為O，將其餘的O改為X即可
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                if(board[i][j] == 'A')
                    board[i][j] 
 
= 'O';
                else if(board[i][j] == 'O')
                    board[i][j] = 'X';
            }
        }
        return;
    }

    void dfs(vector<vector<char>>& board, int i, int j){
        int m = board.size();
        int n = board[0].size();

        if(i>=m||i<0||j>=n||j<0
 
||board[i][j]!='O')
            return;
        board[i][j]='A';
        dfs(board,i+1,j);
        dfs(board,i-1,j);
        dfs(board,i,j+1);
        dfs(board,i,j-1);
    }
};