There are a lot of cyclic dependencies in Doom.
Thinkers maintain pointers to each other so that they can be iterated over in O(n) time
where n is the number of Thinkers actually active. The linked list was required in Doom
due to the way the allocations work.
Thinkers that are map-objects generally maintain a pointer to a subsector they reside in. The subsector also maintains a pointer to a Thinker, and each Thinker maintains a pointer to others forming yet another linked list.