努力将双重链接的通用列表转换为循环。 （段错误）

So I've got this non-circular doubly linked list sample and I find it rather hard to transfer it to a circular one. (segfaulting on the //added lines) Any advise would be nice. Thanks in advance. Also, should I use overloading to handle input of different types? How do I manipulate them? A link with a proper example (really couldn't find one) would make me happy.So I've got this non-circular doubly linked lis