DFS from border O's to mark safe. Flip remaining O's, restore safe ones.
function solve(board): if not board: return m, n = len(board), len(board[0])
def dfs(r, c):
if r < 0 or r >= m or c < 0 or c >= n or board[r][c] != 'O':
return
board[r][c] = 'S'
dfs(r-1, c)
dfs(r+1, c)
dfs(r, c-1)
dfs(r, c+1)
for i in range(m):
dfs(i, 0)
dfs(i, n-1)
for j in range(n):
dfs(0, j)
dfs(m-1, j)
for i in range(m):
for j in range(n):
if board[i][j] == 'O':
board[i][j] = 'X'
elif board[i][j] == 'S':
board[i][j] = 'O'
time.