What is Interview Forge?

Interview Forge helps you practice mock interviews, follow study plans, track career goals, review learning analytics, and prepare across 36 pages.

LRU Cache — Interview Guide

Design an O(1) LRU cache with a hash map plus doubly linked list, the design question that tests data-structure combination skills.

Updated 2026-09MediumDesign

The problem

Design a data structure that supports get(key) and put(key, value) in O(1) average time, with a fixed capacity that evicts the least-recently-used item when full.

Key concepts

A hash map gives O(1) lookup by key.
A doubly linked list ordered by recency gives O(1) eviction and move-to-front.
Store list-node references in the map so you can detach a node without scanning.

Approach

On get: move the accessed node to the head (most recent); return its value.
On put: update if present (and move to head); otherwise insert at head and evict the tail if over capacity.
Use sentinel head/tail nodes to avoid null checks on detach.

Complexity

Operation	Time	Space
get	O(1)	O(capacity)
put	O(1)	O(capacity)

Unlock the full guide

Premium members get every DSA, system design, company and role interview guide — plus AI practice, mock interviews, and analytics.

Unlock with Premium

This problem at 10 companies

Amazon Google Meta Microsoft Apple Netflix NVIDIA Stripe Uber Airbnb

This problem for 9 roles

Senior Frontend Engineer Senior Backend Engineer Staff Engineer Full Stack Engineer AI Engineer AI/FDE (Forward Deployed Engineer)Machine Learning Engineer Engineering Manager New Grad Software Engineer