LeetCode_Surrounded Regions

Given a 2D board containing 'X' and 'O', capture all regions surrounded by 'X'.



A region is captured by flipping all 'O's into 'X's in that surrounded region .

For example,



X X X X

X O O X

X X O X

X O X X

After running your function, the board should be:



X X X X

X X X X

X X X X

X O X X

　　solution： 思想是mark 出所有外围的O，里面的不管。

1. First scan the four edges of the board, if you meet an 'O', call a recursive mark function to mark that region to something else (for example, '+');
2. scan all the board, if you meet an 'O', flip it to 'X';
3. scan again the board, if you meet an '+', flip it to 'O';

   class Solution {
   
   public:
   
        void BFS(vector<vector<char>> &board, int i , int j){
   
       
   
           if(board[i][j] == 'X' || board[i][j] == '#') return;
   
           board[i][j] = '#';
   
     
   
           if( i -1 >= 0 ) BFS(board, i-1, j );
   
           if( i + 1 <  rows  ) BFS(board, i+1, j ); 
   
           if( j -1 >= 0 ) BFS(board, i, j-1 );
   
           if( j +1 < columns ) BFS(board, i, j+1 );
   
       }
   
       void solve(vector<vector<char>> &board) {
   
           // Start typing your C/C++ solution below
   
           // DO NOT write int main() function
   
           rows = board.size();
   
           if( rows == 0) return ;
   
           columns = board[0].size();
   
           if(columns == 0) return ;
   
           
   
           for( int i = 1; i < columns - 1 ; i++)
   
           {
   
               BFS(board, 0,i);//up
   
               BFS(board, rows - 1 , i );//down
   
           }
   
           for( int i = 0; i< rows; i++)
   
           {
   
               BFS(board, i, 0 ); //left
   
               BFS(board, i, columns - 1); //right
   
           }
   
           for(int i = 0; i< rows; i++)
   
               for(int j = 0; j < columns ; j++)
   
               {
   
                 if(board[i][j] == '#') 
   
                    board[i][j] = 'O';
   
                 else if (board[i][j] == 'O')
   
                   board[i][j] = 'X';
   
               }    
   
       }
   
   private :
   
       int rows;
   
       int columns;
   
   };