130. 被围绕的区域

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """
        if not board or not board[0]:
            return
        row = len(board)
        col = len(board[0])

        def dfs(i, j):
            board[i][j] = "B"
            for x, y in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
                tmp_i = i + x
                tmp_j = j + y
                if 1 <= tmp_i < row and 1 <= tmp_j < col and board[tmp_i][tmp_j] == "O":
                    dfs(tmp_i, tmp_j)

        for j in range(col):
            # 第一行
            if board[0][j] == "O":
                dfs(0, j)
            # 最后一行
            if board[row - 1][j] == "O":
                dfs(row - 1, j)

        for i in range(row):
            # 第一列
            if board[i][0] == "O":
                dfs(i, 0)
            # 最后一列
            if board[i][col-1] == "O":
                dfs(i, col - 1)

        for i in range(row):
            for j in range(col):
                # O 变成 X
                if board[i][j] == "O":
                    board[i][j] = "X"
                # B 变成 O
                if board[i][j] == "B":
                    board[i][j] = "O"

# 学习别人的深度搜索
# 链接：https://leetcode-cn.com/problems/surrounded-regions/solution/dfs-bfs-bing-cha-ji-by-powcai/