What is the difference between a hash set and a hash map?

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A hash set stores only keys, while a hash map stores keys paired with values. Both hash each key to place and find entries fast. The difference is what each slot holds.

A set answers one question, whether a key is present. You add keys and later ask whether a key is in the collection. A map answers a richer question, which value goes with a given key.

seen.add("alice");       // set: membership only
ages.set("alice", 30);   // map: key -> value

Use a set when you only care about presence or uniqueness, like tracking which IDs you have already processed. Use a map when each key needs an attached value. Internally, a set is often a map whose values are placeholders.

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Likely follow-ups

How would you remove duplicates from a list using a hash set, and what does it cost? Walk the list once, check membership before adding each item, and name O(n) average time plus O(n) extra space for the set.
You need to count how often each word appears in a text. Set or map, and why? A map from word to count, because a set only records that a word appeared. Mention reading the current count, starting at zero, then writing it back.
What goes wrong if you change an object's fields after adding it to a hash set? If the change alters its hash, lookups check the wrong bucket and miss it. Keep keys immutable, or never change the fields that feed the hash.
When would you choose a tree-based set or map over the hash version? Give sorted iteration or range queries as the reason, and the price, O(log n) per operation instead of O(1) on average.
If a set is built on a map with placeholder values, what does that cost? Time complexity stays the same, but every entry still carries an unused value slot. A set that stores only keys saves that memory.

What you can say

A hash set stores only keys, and a hash map stores each key together with a value.
Both hash the key to decide where an entry goes, so adding and finding entries is fast in either one.
The difference is what each slot holds. A set only tells you whether a key is present, and a map tells you which value goes with that key.
I'd use a set to track which IDs I've already processed, and a map when each key needs data attached, like a user's age.
Many sets are a map with placeholder values inside, so I pick a map only when I'll read the value back. For presence alone, a set says what I mean.

Weak answers to avoid

Says sets and maps hash differently Both hash the key the same way to find its slot. The only difference is what the slot holds, a key alone or a key with its value.
Gives definitions but no use case The interviewer wants to hear when you'd pick each one. Name a case, like tracking processed IDs with a set or storing a count per key in a map.
Claims a set is faster than a map Both hash the key, so lookups cost the same, O(1) on average. A set can save memory per entry. It does not save time.

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