HashMap

Summary

A key-value data structure offering average O(1) lookup, insert and delete — arguably the single most useful tool for optimizing brute-force solutions to O(n).

• 1Solve two-sum using a hash map and compare it to the O(n²) brute-force approach.
• 2Practice using a hash map to count frequencies (anagrams, duplicates).
• 3Learn how hash maps achieve near-constant-time lookups under the hood (hashing + buckets).