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LeetCode 1726 (Medium) is a Google, Amazon, and Meta hashmap interview favorite. Build a frequency map of all pair products, then apply choose-2 combinatorics and a factor of 8 to count ordered tuples.
LeetCode 535 (Medium) is the on-ramp to system design interviews at Google, Amazon, and Microsoft. Build O(1) encode and decode using two hashmaps and a counter — the core data structure behind every URL shortener.
LeetCode 166 (Medium) shows up in Google, Amazon, and Microsoft interviews. Convert a fraction to its decimal string by simulating long division and tracking remainders in a hashmap to detect the repeating cycle.
LeetCode 652 (Medium) is a Google, Amazon, and Microsoft staple. Serialize each subtree during post-order DFS, store the serialization in a hashmap, and report nodes whose serialization first hits a count of two.
Implement a HashMap from scratch using an array of buckets with chaining for collision resolution — the foundational data structure interview that every engineer should be able to implement cold.