How do you find all duplicates in an array where values are in 1 to n and some appear twice?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Treat values as indices: negate nums[abs(x)-1] on first visit and record x when you see a negative already.
WHY — Arrays instead of guessing?
Why interviewers care about Arrays:
question about Arrays.
trade-offs, and what you would actually do on a DSA project - not buzzwords.
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:
- 1Treat values as indices:
negate nums[abs(x)-1] on first visit and record x when you see a negative already.
- 2That is O(n) time
and O(1) extra space besides the answer, mutating the array.
- 3A hash set is
O(n) space if mutation is not allowed.
- 4Give an example
One tiny concrete case you can say aloud.
- 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
Treat values as indices: negate nums[abs(x)-1] on first visit and record x when you see a negative already. That is O(n) time and O(1) extra space besides the answer, mutating the array.