Doubly-Linked List Recap

   prev <-- node --> next
   prev <-- node --> next
   prev <-- node --> next

Each node has prev and next pointers. With a sentinel head + tail, even insertions/removals at boundaries are uniform.

Both operations are O(1). The "sentinel" head/tail nodes mean we don't need null checks at the boundaries.

For LRU: the most-recently-used is at the FRONT. The LRU (eviction candidate) is at the BACK (just before tail).