Easy Hashing Question 16 of 224

How do you solve Two Sum in linear time?

DSA interview set · Speak this in 60–90 seconds · Faridabad & Delhi NCR

PICTURE THIS: DATABASE INDEX

Without indexScan every row

With indexJump to keys

CostWrites slower

Simple meaning

While scanning, store each value and its index in a hash map and look up target minus the current number before inserting.

WHY — Hashing instead of guessing?

Why interviewers care about Hashing:

This is a process

question about Hashing.

Panels listen for order,

trade-offs, and what you would actually do on a DSA project - not buzzwords.

Stay structured

Name the idea, why it exists, then one short example.

Close cleanly

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:

1
While scanning, store each
value and its index in a hash map and look up target minus the current number before inserting.
2
Average time is O(n)
with O(n) space.
3
Sorting plus two pointers
is O(n log n) and loses original indices unless you keep them, so hashing is the usual interview answer.
4
Give an example
One tiny concrete case you can say aloud.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

EXAMPLE — See it in action

Here's a short line you can speak, broken into clear beats:

Say this line

“Average time is O(n) with O(n) space.”

Break into beats

AveragetimeisOnwith

Speaking order

2987408337471632900

Note: Adapt this scaffold to your own project — keep it under 60–90 seconds.

Key takeaway

While scanning, store each value and its index in a hash map and look up target minus the current number before inserting. Average time is O(n) with O(n) space.