What is the time complexity for adding, removing, or searching for an element in a HashSet<T>?

Short answer: All these operations generally have O(1) average time complexity due to the underlying hash table structure.

Real-world example (ShopNest)

When applying a coupon, ShopNest keeps used coupon codes in a HashSet<string> so “already used?” checks stay fast and unique.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.