小火车挂车厢

# 小火车挂车厢：用类实现单链表
class Car:
    """一节车厢：装着车厢号，挂着下一节车厢"""
    def __init__(self, no):
        self.no = no
        self.next = None

class Train:
    """一列小火车：head 指向第一节车厢"""
    def __init__(self):
        self.head = None

    def add_tail(self, no):
        """在车尾挂一节车厢"""
        new_car = Car(no)
        if self.head is None:
            self.head = new_car
            return
        cur = self.head
        while cur.next is not None:
            cur = cur.next
        cur.next = new_car

    def show(self):
        """从头到尾报出车厢号"""
        cars = []
        cur = self.head
        while cur is not None:
            cars.append(str(cur.no))
            cur = cur.next
        print("列车：", " -> ".join(cars))

    def remove(self, no):
        """摘掉编号为 no 的车厢"""
        dummy = Car(0)          # 哨兵车厢，让删第一节也好写
        dummy.next = self.head
        cur = dummy
        while cur.next is not None:
            if cur.next.no == no:
                cur.next = cur.next.next
                break
            cur = cur.next
        self.head = dummy.next

train = Train()
for no in ["W1", "W2", "W3"]:
    train.add_tail(no)
    train.show()

train.remove("W2")
print("摘掉 W2 后：")
train.show()

train.add_tail("W9")
print("挂上 W9 后：")
train.show()