leetcode[130] Surrounded Regions

给定一个类似棋盘，有X和O，把X圈住的O变为X例如：

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

其实这题的思路，就是先找到四周的O，然后和四周的O相邻的O全都是不可能转化为X的，我们用另外一个符号标记，那么最后，矩阵中的O就是被圈住的，就需要转化为X，然后其他标记的符号就要转为原来的O。这个叫做DFS。

class Solution 
{
    public:
    void dfs(vector<vector<char> > &board, int x, int y)
    {
        if(x<0 || x>=board.size() || y<0 || y>=board[0].size() || board[x][y]!='O') return;
        board[x][y]='#';
        dfs(board, x-1,y);
        dfs(board, x+1,y);
        dfs(board, x,y-1);
        dfs(board, x,y+1);
    }
    
    void solve(vector<vector<char> > &board)
    {
        if (board.size() < 3) return ;
        if (board[0].size() < 3) return ;
        int m = board.size(), n = board[0].size();
        for(int j=0;j<n;j++)
        {
            dfs(board,0,j);
            dfs(board,m-1,j);
        }
        for(int i=1;i<m-1;i++)
        {
            dfs(board,i,0);
            dfs(board,i,n-1);
        }
        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++)
            {
                if(board[i][j]=='O') board[i][j]='X';
                else if(board[i][j]=='#') board[i][j]='O';
            } 
    }
};

代码就是对上下左右每个边开始DFS解决。可是会栈溢出，哎，这个error略高级，一直摸不清。

如果把DFS的函数改为如下的形式，就可以AC了。

不同之处就是上面的dfs中判断了上下左右，而下面这个的上下左右是在主函数里，dfs是对内层处理。

class Solution 
{
    public:
    void dfs(vector<vector<char>> &board, int i, int j)
    {
        int row = board.size(), col = board[0].size();
        if(i > 1 && board[i-1][j] == 'O')
        {
            board[i-1][j] = '#';
            dfs(board, i-1, j);
        }
        if(i < row-1 && board[i+1][j] == 'O')
        {
            board[i+1][j] = '#';
            dfs(board, i+1, j);
        }
        if(j > 1 && board[i][j-1] == 'O')
        {
            board[i][j-1] = '#';
            dfs(board, i, j-1);
        }
        if(j < col-1 && board[i][j+1] == 'O')
        {
            board[i][j+1] = '#';
            dfs(board, i, j+1);
        }
    }
    
    void solve(vector<vector<char> > &board)
    {
        if (board.size() < 3) return ;
        if (board[0].size() < 3) return ;
        int m = board.size(), n = board[0].size();
        for(int j=0;j<n;j++)
        {
            if (board[0][j] == 'O')
            {
                board[0][j] = '#';
                dfs(board,0,j);
            }
            if (board[m-1][j] == 'O')
            {
                board[m-1][j] = '#';
                dfs(board,m-1,j);
            }
        }
        for(int i=1;i<m-1;i++)
        {
            if (board[i][0] == 'O')
            {
                board[i][0] = '#';
                dfs(board,i,0);
            }
            if (board[i][n-1] == 'O')
            {
                board[i][n-1] = '#';
                dfs(board,i,n-1);
            }
        }
        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++)
            {
                if(board[i][j]=='O') board[i][j]='X';
                else if(board[i][j]=='#') board[i][j]='O';
            } 
    }
};

如果利用BFS的话：

BFS就是每次处理的时候对该元素的四周的进行处理，而不是和上面所述一样进行元素的一边处理到返回为止。这里用BFS就不会出现溢出的问题。

我们用pair来存x和y的值，注意可以用make_pair()来获取pair对。

如下：

 

class Solution 
{
    public:
    //130 bfs
    void bfs130(vector<vector<char> > &board, int x, int y)
    {
        if(x<0 || x>=board.size() || y<0 || y>=board[0].size() || board[x][y]!='O') return;
        board[x][y] = '#';
        queue<pair<int, int> > que;
        que.push(make_pair(x, y)); // the use of make_pair
        while(!que.empty())
        {
            pair<int, int> tmpPair = que.front();
            que.pop();
            int tmpx = tmpPair.first, tmpy = tmpPair.second;

            if (tmpx > 0 && board[tmpx - 1][tmpy] == 'O') // up
            {
                board[tmpx - 1][tmpy] = '#'; que.push(make_pair(tmpx-1, tmpy));
            }
            if (tmpx < board.size()-1 && board[tmpx + 1][tmpy] == 'O') // down
            {
                board[tmpx + 1][tmpy] = '#'; que.push(make_pair(tmpx+1, tmpy));
            }
            if (tmpy > 0 && board[tmpx][tmpy - 1] == 'O') // left
            {
                board[tmpx][tmpy - 1] = '#'; que.push(make_pair(tmpx, tmpy-1));
            }
            if (tmpy < board[0].size()-1 && board[tmpx][tmpy + 1] == 'O') // right
            {
                board[tmpx][tmpy + 1] = '#'; que.push(make_pair(tmpx, tmpy+1));
            }
        }
        return ;
    }
    void solve(vector<vector<char> > &board)
    {
        if (board.size() < 3) return ;
        if (board[0].size() < 3) return ;
    
        for (int j = 0; j < board[0].size(); ++j)
        {
            bfs130(board, 0, j);
            bfs130(board, board.size()-1, j);
        }
        for (int i = 1; i < board.size()-1; ++i) // i == 0 && i == board.size()-1  has been done before
        {
            bfs130(board, i, 0);
            bfs130(board, i, board[0].size()-1);
        }
    
        for (int i = 0; i < board.size(); ++i)
            for (int j = 0; j < board[0].size(); ++j)
            {
                if (board[i][j] == 'O')
                    board[i][j] = 'X';
                else if (board[i][j] == '#')
                    board[i][j] = 'O';
            }
    }
};