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

"""

m = len(board)

if m < 3:

return

n = len(board[0])

if n < 3:

return

def dfs(i, j):

for (dx, dy) in ((0, 1), (0, -1), (1, 0), (-1, 0)):

x, y = i + dx, j + dy

if 0 <= x < m and 0 <= y < n and board[x][y] == "O":

board[x][y] = "B"

dfs(x, y)

for i in range(m):

for j in range(n):

if (i == 0 or j == 0 or i == m - 1 or j == n - 1) and board[i][j] == "O":

board[i][j] = "B"

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] == "B":

s = Solution()

for board, expect in [

([["X", "X", "X", "X"], ["X", "O", "O", "X"], ["X", "X", "O", "X"], ["X", "O", "X", "X"]],

[["X", "X", "X", "X"], ["X", "X", "X", "X"], ["X", "X", "X", "X"], ["X", "O", "X", "X"]]),

([["O", "O", "O"], ["O", "O", "O"], ["O", "O", "O"]], [["O", "O", "O"], ["O", "O", "O"], ["O", "O", "O"]]),

]:

s.solve(board)