| from typing import List | |
| class Solution: | |
| def solve(self, board: List[List[str]]) -> None: | |
| if not board or not board[0]: | |
| return | |
| m, n = len(board), len(board[0]) | |
| def dfs(x: int, y: int): | |
| if x < 0 or x >= m or y < 0 or y >= n or board[x][y] != "O": | |
| return | |
| board[x][y] = "B" | |
| dfs(x - 1, y) | |
| dfs(x + 1, y) | |
| dfs(x, y - 1) | |
| dfs(x, y + 1) | |
| 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] == "B": | |
| board[i][j] = "O" | |