LeetCode OJ - Surrounded Regions

题目：

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

解题思路：

　　参照网上的解题思路：先从四周的‘O'进行搜索，将能到达的'O'换成第三种符号，如（'#')， 再遍历所有符号把O换成X，第三种符号换回O。

代码：

class Solution {
public:  
    int m, n;
    
    void traversal(int x, int y, vector<vector<char>> &board) {
        if (x >= 0 && x < m && y >= 0 && y < n && board[x][y] == 'O') {
            board[x][y] = 'Z';
            traversal(x - 1, y, board);
            traversal(x + 1, y, board);
            traversal(x, y - 1, board);
            traversal(x, y + 1, board);
        }    
    }
    
    void solve(vector<vector<char>> &board) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (board.empty() || board.size() == 0 || board[0].size() == 0) {
            return;
        }
        m = board.size(), n = board[0].size();
        for (int i = 0; i < n; i++) {
            traversal(0, i, board);
            traversal(m - 1, i, board);
        }
        for (int i = 0; i < m; i++) {
            traversal(i, 0, board);
            traversal(i, n - 1, board);
        }
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                board[i][j] = board[i][j] == 'Z' ? 'O' : 'X';
            }
        }
    }
};