How do you find the longest consecutive sequence of integers in an unsorted array?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Put all numbers in a set, then only start a streak from values that have no predecessor x-1, and walk x+1, x+2, ...
WHY — Hashing instead of guessing?
Why interviewers care about Hashing:
question about Hashing.
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:
- 1Put all numbers in
a set, then only start a streak from values that have no predecessor x-1, and walk x+1, x+2, ...
- 2Why it exists
while present.
- 3Each number is visited
a constant number of times: O(n) average time and O(n) space.
- 4Sorting first is O(n
log n) and is the simpler backup.
- 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
Put all numbers in a set, then only start a streak from values that have no predecessor x-1, and walk x+1, x+2, ... while present.