面试题41：包围区域

　　包围区域是一个典型的DFS的题目。大意是一个二维数组中存在字符'X'和'O'，将被'X'包围的'O'全部变为'X'。具体做法是从数组的边界上为'O'的起点开始DFS，将DFS过程中遇到的字符标记为'Y'。然后遍历二维数组，将'Y'的变为'O'，其余为'X'即可。

题目描述：

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

class Solution {
public:
    void dfs(int x,int y,vector<vector<char>>& board){
        if(x<0 || x >= board.size()) return;
        if(y<0 || y >= board[0].size()) return;
        if(board[x][y] == 'O'){
            board[x][y] = 'Y';
        }else{
            return;
        }
        dfs(x+1,y,board);
        dfs(x-1,y,board);
        dfs(x,y+1,board);
        dfs(x,y-1,board);

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

        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(board[i][j] == 'Y'){
                    board[i][j] = 'O';
                }else{
                    board[i][j] = 'X';
                }
            }
        }
    }
};