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

思路1：采用DFS遍历
    从矩阵最外围每个边一次进行DFS遍历，如果值为'O'则把值标记为‘T',并向上下左右四个
    方向遍历，当四个方向遍历结束后，遍历整个矩阵，如果值为'O'则标记为'X'，如果值为
    'T'则标记为'O';
*/

class Solution {
public:
    void solve(vector<vector<char>>& board) {
        if(board.empty()) return;
        int m= board.size(),n= board[0].size();
        for(int i=0;i<m;i++){
            dfs(board,i,n-1);
            dfs(board,i,0);
        }
        for(int j=0;j<n;j++){
            dfs(board,0,j);
            dfs(board,m-1,j);
        }
        for(int i=0;i<m;i++)
            for(int j=0;j<n;j++){
                if(board[i][j]=='T') board[i][j] = 'O';
                else if(board[i][j]=='O') board[i][j] = 'X';
            }
    }
    void dfs(vector<vector<char>>& board,int i,int j){
        if(board[i][j] == 'O'){
            if(board[i][j]=='O') board[i][j] = 'T';
            if(j-1 > 0 ) dfs(board,i,j-1);
            if(j+1 < board[0].size()) dfs(board,i,j+1);
            if(i-1 > 0 ) dfs(board,i-1,j);
            if(i+1 < board.size()) dfs(board,i+1,j);
        }   
    }
};