How does quicksort work, and what is its average versus worst-case complexity?
PICTURE THIS: 1, 2, 2, 8
Simple meaning
Pick a pivot, partition smaller keys left and larger right, then recurse.
WHY — Sorting instead of guessing?
Why interviewers care about Sorting:
contrast on Sorting, not two memorised paragraphs.
the developer, then one case where picking wrong hurts.
Name the idea, why it exists, then one short example.
End with when you use it and one common pitfall.
STEPS — What happens step by step?
Before you speak the answer, walk the interviewer through these steps:
- 1Pick a pivot, partition
smaller keys left and larger right, then recurse.
- 2Average time is O(n
log n) with O(log n) stack space
- 3worst case is O(n^2)
on already-sorted data with a bad pivot.
- 4Random or median-of-three pivots
make the worst case unlikely.
- 5Common mistake
What juniors usually get wrong.
- 6Close
When you pick this over the alternative.
EXAMPLE — See it in action
Here's a short line you can speak, broken into clear beats:
Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.
Key takeaway
Pick a pivot, partition smaller keys left and larger right, then recurse. Average time is O(n log n) with O(log n) stack space