1 class Solution { 2 public: 3     void solve(vector<vector<char>> &board) { 4         int m = board.size(); 5         if(m == 0) 6             return; 7         int n = board[0].size(); 8         int i,j; 9         for(i=0;i<m;i++)  10         {  11             if (board[i][0]==‘O‘)  12                 bfs(i,0,board);  13             if (board[i][n-1]==‘O‘)  14                 bfs(i,n-1,board);  15         }  16         for(j=0;j<n;j++)  17         {  18             if (board[0][j]==‘O‘)  19                 bfs(0,j,board);  20             if (board[m-1][j]==‘O‘)  21                 bfs(m-1,j,board);  22         }  23         for(i=0;i<m;i++)  24         {25             for(j=0;j<n;j++)  26             {  27                 if(board[i][j]==‘O‘)  28                     board[i][j]=‘X‘;  29                 else if (board[i][j]==‘E‘)  30                     board[i][j]=‘O‘;  31             }  32         }33     }34     void bfs(int i,int j,vector<vector<char>> &board){35         int m = board.size();36         int n = board[0].size();37         38         queue<pair<int,int> > Q;39         Q.push(make_pair(i,j));40         board[i][j] = ‘E‘;41         int dx[4] = {-1,0,0,1};42         int dy[4] = {0,-1,1,0};43         while(!Q.empty()){44             pair<int,int> p = Q.front();45             Q.pop();46             47             int iidx = p.first;48             int jidx = p.second;49             50             for(int i = 0 ; i < 4 ; ++i){51                 int itmp = iidx+dx[i];52                 int jtmp = jidx+dy[i];53                 54                 if((itmp < m && itmp >= 0)&&(jtmp < n && jtmp >= 0) && board[itmp][jtmp] == ‘O‘){55                     Q.push(make_pair(itmp,jtmp));56                     board[itmp][jtmp] = ‘E‘;57                 }58             }59         }60     }61 };