This is about some data structure
DulLinkList is a data structure,that can store data in LinkList,if you wanna insert or delete a data in LinkList,you just change your pointer,and in DulLinkList,The 'prior' pointer point to the former of the current ADT(Abstract Data Type),and 'next' pointer point the latter of the current ADT.and the head ADT don't save data,if you want you can store length of the LinkList.