'O' 相连的 'O' 最终都会被填充为 'X'。如果两个元素在水平或垂直方向相邻，则称它们是“相连”的。
示例 2：

输入：board = [["X"]]
输出：[["X"]]
 

提示：

m == board.length
n == board[i].length
1 <= m, n <= 200
board[i][j] 为 'X' 或 'O'


链接：https://leetcode-cn.com/problems/surrounded-regions


class Solution:
    def solve(self, board: List[List[str]]) -> None:


class Solution:
    def solve(self, borad: List[List[str]]) -> None:
        row, col = len(borad), len(board[0])

        def dfs(x, y):
            if board[x][y] == 'X':
                return 
            else:
               board[x][y] = '#'

            for c in [(0,1),(0,-1),(-1,0),(-1,1)]:
                nx, ny = x + c[0], y + c[1]
                if 0 <= nx < row and 0 <= ny < col:
                    dfs(nx, ny)

        for i in range(row):
            dfs(i, 0)
            dfs(i, col-1)

        for j in range(1, col-1):
            dfs(0, j)
            dfs(row-1, j)
        
        for i in range(row):
            for j in range(col):
                if board[x][y] == '#':
                    board[x][y] = 'O'
                else:
                    board[x][y] = 'X'