小停车场有 spaces 个车位。写完 arrive(plate)：还有空位才让车进来（车牌放进 self.cars），满了就不进。leave(plate)：车在里面才拿掉，不在就什么也不做。
still_parked(2, [("in", "A1"), ("in", "B2"), ("in", "C3")])→['A1', 'B2']still_parked(2, [("in", "A1"), ("out", "A1"), ("in", "C3")])→['C3']class ParkingLot:
def __init__(self, spaces):
self.spaces = spaces
self.cars = []
def arrive(self, plate):
# 在这里写：还有空位，就把车牌放进 self.cars
pass
def leave(self, plate):
# 在这里写：车牌在 self.cars 里，就把它拿掉
pass
def still_parked(spaces, events):
lot = ParkingLot(spaces)
for action, plate in events:
if action == "in":
lot.arrive(plate)
else:
lot.leave(plate)
return sorted(lot.cars)
写好了先点「运行」试试示例，没问题再点「提交」。