[LeetCode]130 Surrounded Regions（DFS）

题目链接：https://leetcode.com/problems/surrounded-regions/?tab=Description

题意：把非边界的O改成X。

先dfs边界的，打好标记。把没变的变成X后同时更新标记的为O。

class Solution {
public:
    const int dx[5] = {0, 0, 1, -1};
    const int dy[5] = {1, -1, 0, 0};
    int n, m;
    bool ok(int x, int y) {
        return x >= 0 && x < n && y >= 0 && y < m;
    }
    void dfs(vector<vector<char>>& board, int x, int y) {
        board[x][y] = 'Q';
        for(int i = 0; i < 4; i++) {
            int xx = x + dx[i];
            int yy = y + dy[i];
            if(ok(xx, yy) && board[xx][yy] == 'O') {
                dfs(board, xx, yy);
            }
        }
    }
    void solve(vector<vector<char>>& board) {
        n = board.size();
        if(n == 0) return;
        m = board[0].size();
        for(int i = 0; i < n; i++) {
            if(board[i][0] == 'O') dfs(board, i, 0);
            if(board[i][m-1] == 'O') dfs(board, i, m-1);
        }
        for(int i = 0; i < m; i++) {
            if(board[0][i] == 'O') dfs(board, 0, i);
            if(board[n-1][i] == 'O') dfs(board, n-1, i);            
        }
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < m; j++) {
                if(board[i][j] == 'O') board[i][j] = 'X';
                else if(board[i][j] == 'Q') board[i][j] = 'O';
            }
        }
    }
};