Advent of code 2024 day 18 brute-force

def day18(s, *, size=71, n_step=1024, part2=False):  # Brute-force.
  blocks = np.array([row.split(',') for row in s.splitlines()], int)[:, ::-1] + 1
  grid = np.pad(np.full((size, size), '.'), 1, constant_values='#')
  grid[tuple(blocks[:n_step].T)] = '#'

  def get_distance() -> int | None:
    yx, yxl = (1, 1), (size, size)
    visited = {yx}
    lst = [yx]
    for distance in itertools.count():
      lst2 = []
      for yx in lst:
        if yx == yxl:
          return distance
        y, x = yx
        for dy, dx in ((0, -1), (0, 1), (-1, 0), (1, 0)):
          yx2 = y + dy, x + dx
          if grid[yx2] == '.' and yx2 not in visited:
            visited.add(yx2)
            lst2.append(yx2)
      lst = lst2
      if not lst:
        return None

  if not part2:
    return get_distance()

  for y, x in blocks[n_step:]:
    grid[y, x] = '#'
    if get_distance() is None:
      return f'{x - 1},{y - 1}'