Interview Questions

Average case O(1) for insert, lookup, and delete thanks to hashing. Worst case O(n) when many keys collide into one bucket (degenerate hashing). A good hash function and load-factor-based resizing keep operations near O(1).

Use Floyd's tortoise-and-hare: advance one pointer by 1 and another by 2 each step. If they ever meet, there's a cycle; if the fast pointer reaches null, there isn't. O(n) time, O(1) space.