Solution.py

class Solution:
    def solve(self, board: List[List[str]]) -> None:
        """
        Do not return anything, modify board in-place instead.
        """

        def dfs(i, j):
            board[i][j] = '.'
            for a, b in [[0, -1], [0, 1], [1, 0], [-1, 0]]:
                x, y = i + a, j + b
                if 0 <= x < m and 0 <= y < n and board[x][y] == 'O':
                    dfs(x, y)

        m, n = len(board), len(board[0])
        for i in range(m):
            for j in range(n):
                if board[i][j] == 'O' and (
                    i == 0 or i == m - 1 or j == 0 or j == n - 1
                ):
                    dfs(i, j)
        for i in range(m):
            for j in range(n):
                if board[i][j] == 'O':
                    board[i][j] = 'X'
                elif board[i][j] == '.':
                    board[i][j] = 'O'