1582 articles
LRU Cache is one of the most important design problems in tech interviews — it combines a HashMap for O(1) lookup with a doubly linked list for O(1) eviction order. Amazon, Google, Meta, and Microsoft use it to assess system design thinking at the data structure level.
Subarray Sum Equals K is the definitive prefix-sum hash map problem. It teaches the pattern of converting a range-sum query into a complement lookup — reducing O(n^2) to O(n). Amazon, Google, and Meta ask this in nearly every data-focused interview loop.
Continuous Subarray Sum applies the modular prefix sum trick — one of the most elegant applications of number theory to hash map design. Google uses this problem to test whether candidates can combine modular arithmetic with hash-map complement lookup.
Longest Consecutive Sequence is a deceptively hard problem that Google and Meta use to test whether candidates can achieve O(n) without sorting. The key insight — only start counting from sequence beginnings — turns an O(n^2) brute force into an O(n) HashSet solution.
Insert Delete GetRandom O(1) is a classic design interview problem that Google, Amazon, and Meta use to assess compound data structure thinking. The trick — swapping the target with the last element before deletion — enables O(1) removal from a dynamic array.