How do you find two indices of numbers that add to a target when the array is unsorted and you must not sort?
PICTURE THIS: ARRAY IN MEMORY
Index starts at 0. Scan once for max — O(n).
Simple meaning
Same as Two Sum: one pass with value-to-index map, looking up target-x before inserting x so you never reuse the same index.
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:
- 1Same as Two Sum:
one pass with value-to-index map, looking up target-x before inserting x so you never reuse the same index.
- 2Average O(n) time and
O(n) space.
- 3Sorting would be O(n
log n) and would scramble indices unless you store pairs.
- 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
Same as Two Sum: one pass with value-to-index map, looking up target-x before inserting x so you never reuse the same index. Average O(n) time and O(n) space.