Hashing: Hash Maps, Sets and Collisions

The hash map is the most-used data structure in coding interviews and the least-understood. "It is O(1)" is where most explanations stop, which leaves the two questions interviewers actually ask — why is it only average-case, and what happens on collision — without an answer.

These lessons open the structure up. You watch keys hash to buckets, watch collisions occur and get resolved, and see why the average case degrades. Frequency counting is covered as its own lesson because it is the single most common application of hashing in interview problems.