### solve 
from typing import *
def f_gold(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'