130被围绕的区域

from typing import List
# 这道题的思路是从矩阵的四周开始寻找，如果这个字符为O，就向上下左右递归，
# 先将找到的O转化为其他字符，最后将没有找到的O转化为X，将找到的O不变
class Solution:
    def solve(self, board: List[List[str]]) -> None:
        # 如果列表为空或者列表中为空，返回空
        if not board or not board[0] :return
        # 定义行和列
        row,col = len(board),len(board[0])
        # 定义递归函数
        def dfs(index1, index2):
            # 首先将进来的转化为其他字符
            board[index1][index2] = "B"
            # 然后上下左右递归
            for x, y in [(0, 1), (0, -1), (1, 0), (-1, 0)]:
                # 注意这里不能直接在index1上边相加减
                i = index1 +  x
                j = index2 +  y
                # 注意这里一定是从1开始的，否则就是错误的。
                if 1 <= i < row and 1 <= j < col and board[i][j] == "O":
                    dfs(i,j)
        # 寻找第一列和最后一列
        for i in range(row):
            if board[i][0] == "O":
                dfs(i,0)
            if board[i][col - 1] == "O":
                dfs(i,col - 1)
        # 寻找第一行和最后一行
        for i in range(col):
            if board[0][i] == "O":
                dfs(0,i)
            if board[row - 1][i] == "O":
                dfs(row - 1,i)
        # 将字符恢复原位。
        for i in range(row):
            for j in range(col):
                if board[i][j] == "O":
                    board[i][j] = "X"
                if board[i][j] == "B":
                    board[i][j] = "O"


A = Solution()
A.solve([["X","X","X","X"],["X","O","O","X"],["X","X","O","X"],["X","O","X","X"]])