要求：把被围起来的o改成x但是o在边界的话全都不能改
思路：dfs，当然bfs和并查集也行，用了个数组存了改过的元素发现有o在边界的全部改回来，一遍过了

class Solution {
public:
    void dfs(vector<vector<char>>& board,vector<pair<int,int>> &o,int i,int j,bool &addright){
        if(i<0||i>=board.size()||j<0||j>=board[0].size()||board[i][j]=='X')return;
        if(i==0||i+1==board.size()||j==0||j+1==board[0].size()){
            addright=false;
            return;
        }
        board[i][j]='X';
        o.push_back(make_pair(i,j));
        int di[4]={0,0,1,-1};
        int dj[4]={1,-1,0,0};
        for(int k=0;k<4;++k)
            dfs(board,o,i+di[k],j+dj[k],addright);
    }
    void solve(vector<vector<char>>& board) {
        for(int i=1;i+1<board.size();++i)
            for(int j=1;j+1<board[0].size();++j)
                if(board[i][j]=='O'){
                    vector<pair<int,int>> o;
                    bool addright=true;
                    dfs(board,o,i,j,addright);
                    if(!addright)
                        for(auto p:o)
                            board[p.first][p.second]='O';
                }
    }
};