说明：  队列是一种特殊的线性表，它只允许在表的前端（front）进行删除操作，而在表的后端（rear）进行插入操作。进行插入操作的端称为队尾，进行删除操作的端称为队头。队列中没有元素时，称为空队列。
(Queue is a special kind of linear form, it only allows the front of the table (front) to delete, and in the form of back-end (rear) to insert. The insertion of the tail end is called, the deletion of the terminal as team head. There is no queue element, called the empty queue.)