查看两个链表是否相等的算法

I have an algorithm which compares two linked lists, L1 and L2, and the elements that are equal, it puts them in a third list L3 while deleting them from L2. I am not sure though if this is all what the algorithm is supposed to do, and I don't understand some concepts such as "p1prec" for example. I want to know the design of algorithm rationally, the bigger picture. When I try to get the sense of it by examining the instructions one by one I feel like Im trying to memorize it. Also some advices on methods to design alike algorithms would be quite welcome.The algorithm is as follows :I have an algorithm which compares two linked l