130. Surrounded Regions

Given a 2D board containing 'X' and 'O' (the letter 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

DFS的时候用两个list存访问过的地址，如果没有touch过边界，则用这两个list的值来给board改写。但是这个耗费的space比较大，过不了OJ的大的test case。

public void Solve(char[,] board) {
        int row = board.GetLength(0);
        int col = board.GetLength(1);
        bool[,] visited = new bool[row,col];
        
        for(int i =0;i< row;i++)
        {
            for(int j =0;j<col;j++)
            {
                if(board[i,j] == 'O' && !visited[i,j])
                {
                    List<int> rowStore = new List<int>();
                    List<int> colStore = new List<int>();
                    bool NotWrap= BackTracking(board,i,j,row,col,visited,rowStore,colStore);
                    if(!NotWrap)
                    {
                        for(int m = 0;m< rowStore.Count();m++)
                        {
                            board[rowStore[m],colStore[m]] ='X';
                        }
                    }
                }
            }
        }
        return;
    }
    
    private bool BackTracking(char[,] board, int x, int y, int row, int col, bool[,] visited, List<int> rowStore, List<int> colStore)
    {
        if(x<0 || x> row-1) return true;
        if(y<0 || y> col-1) return true;
        if(visited[x,y]) return false;
        if(x == 0 || x == row-1)
        {
            visited[x,y] = true;
            if(board[x,y] == 'O') 
                return true;
        }
        if(y == 0 || y == col-1)
        {
            visited[x,y] = true;
            if(board[x,y] == 'O') return true;
        }
      
        if(board[x,y] == 'X') return false;
        visited[x,y] = true; 
        rowStore.Add(x);
        colStore.Add(y);
        return BackTracking(board,x+1,y,row,col,visited,rowStore,colStore) || BackTracking(board,x-1,y,row,col,visited,rowStore,colStore) || BackTracking(board,x,y+1,row,col,visited,rowStore,colStore) || BackTracking(board,x,y-1,row,col,visited,rowStore,colStore);
    }
    
    

用边界点为‘O’的开始做DFS， 找到的做标记‘#’。然后第二次遍历，标记为‘#’的全换位‘O’, 标记为‘O’的换位‘X’.

 public void Solve(char[,] board) {
        int row = board.GetLength(0);
        int col = board.GetLength(1);
        
        for(int i =0;i< row;i++)
        {
            for(int j =0;j<col;j++)
            {
                if(((i== 0 || i== row-1) ||(j == 0|| j ==col-1))&&(board[i,j] == 'O'))
                {
                    BackTracking(board,i,j,row,col);
                }
            }
        }
        
        for(int i =0;i< row;i++)
        {
            for(int j =0;j<col;j++)
            {
                if(board[i,j] == 'O') board[i,j] = 'X';
                else if(board[i,j] == '#') board[i,j] = 'O';
            }
        }
        return;
    }
    
    private void BackTracking(char[,] board, int x, int y, int row, int col)
    {

        //if(board[x,y] != 'O') return ;
        
        board[x,y] ='#';
        
        if(x<row-1 && board[x+1,y]=='O') BackTracking(board,x+1,y,row,col);
        if(x>0 && board[x-1,y]=='O') BackTracking(board,x-1,y,row,col);
        if(y<col-1 && board[x,y+1]=='O') BackTracking(board,x,y+1,row,col);
        if(y>0 && board[x,y-1]=='O') BackTracking(board,x,y-1,row,col);
    }