How do you find the smallest missing positive integer in an unsorted array?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Place each value v in index v-1 when it lies in 1..n, then scan for the first index i whose value is not i+1.
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:
- 1Place each value v
in index v-1 when it lies in 1..n, then scan for the first index i whose value is not i+1.
- 2That cyclic placement is
O(n) time and O(1) extra space.
- 3A hash set of
positives is O(n) space
- 4sorting is O(n log
n) and usually not the intended answer.
- 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
Place each value v in index v-1 when it lies in 1..n, then scan for the first index i whose value is not i+1. That cyclic placement is O(n) time and O(1) extra space.