Q3.7 Create a data structures to hold dogs and cats.（待续）

Q: An animal shelter holds only dogs and cats, and operates on a strictly "first in, first out" basis. People must adopt either the "oldest" (based on arrival time) of all animals at the shelter, or they can select whether they would prefer a dog or a cat
(and will receive the oldest animal of that type). They cannot select which specific animal they would like. Create the data structures to maintain this system and implement operations such as enqueue, dequeueAny, dequeueDog and dequeueCat. You may use the
built-in L inkedL ist data structure.

A: 因为要保存cat和dog，以及其共同体，所以利用虚函数效果更好一些。

首先定义一个新的基本数据结构用来保存动物的名字和入住的顺序。并且用两个链表，分别用来继续dog和cat的数据。

enqueue：设定其order， 再将其加入相应的链表当中

dequeueAny：根据dog和cat第一个数据的顺序，判断返回哪一个数据