Combining: Hash Map + DLL

The classic O(1) LRU = hash map (key -> Node) + doubly-linked list (recency order).

Both get and put are O(1):

• Hash map lookup: O(1)
• DLL remove + add_front: O(1) (we have direct node pointers, no scan)
• Eviction: O(1) (tail.prev is always the LRU)

This is THE classic LRU. Almost every production cache uses this structure.