LRU Cache (with LFU / Custom Eviction Follow-ups)

Problem Overview

Three first-round interviews asked for an LRU cache (two are described as phone screens, the third as a first round). One of them, a Citadel Securities phone-interview report, also lists an LFU cache and a customized evict function (a commenter asked whether all three came in one phone screen; there…

• 2 candidate-reported follow-ups — the exact probes interviewers asked, with the trigger for each
• The rest of the problem statement — full requirements, constraints, and edge cases
• Approach and trade-offs — what passing candidates did, and the mistakes that sink people