Question

I got an interview question saying I need to store few millions of cache and then I need to keep a track on 20 oldest cache and as soon as the threshold of cache collection increases, replace the 20 oldest with next set of oldest cache.

I answered to keep a hashmap for it, again the question increases
what if we wanna access any of the element on hashmap fastly, how to
do, so I told its map so accessing won’t be time taking but
interviewer was not satisfied. So what should be the idle way for such
scenarios.

Answer 1

A queue is well-suited to finding and removing the oldest members.

A queue implemented as a doubly linked list has O(1) insertion and deletion at both ends.

A priority queue lends itself to giving different weights to different items in the queue (e.g. some queue elements may be more expensive to re-create than others).

You can use a hash map to hold the actual elements and find them quickly based on the hash key, and a queue of the hash keys to track age of cache elements.