130. Surrounded Regions (Graph; DFS)

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

思路：从四条边上的'0'入手，与其邻接的'O'都不被改，记为'D'。逆向思维！从不被改的'O'入手，而非寻找被改得'O'

class Solution {
public:
    void dfs(int x, int y){
        if(x< || x>=m || y< || y>=n || board[x][y]!='O') return;
        board[x][y]='D';

        //图的四个方向遍历
        dfs(x-,y);
        dfs(x+,y);
        dfs(x,y-);
        dfs(x,y+);
    }

    void solve(vector<vector<char>> &board){
        if (board.empty()) return;
        this->board = board;
        m=board.size();
        n=board[].size();
        if(n<= || m<=) return;

        for(int j=;j<n;j++){
            dfs(,j);
            dfs(m-,j);
        }

        for(int i=;i<m;i++){
            dfs(i,);
            dfs(i,n-);
        }

        for(int i=;i<m;i++)
            for(int j=;j<n;j++){
                if(this->board[i][j]=='O') this->board[i][j]='X';
                else if(this->board[i][j]=='D') this->board[i][j]='O';
            }
      board = this->board;
    }
private:
    int m,n;
    vector<vector<char>> board;
};