(M)BFS:130. Surrounded Regions

这道题用的是DFS。

扫面矩阵的四条边，如果有O，则用DFS遍历，将所有连着的O都变成另一个字符#。这样剩下是O的就是被X包围着的O，都应该变成X。最后把#再变回O就可以了。

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