Moderate Hashing Question 81 of 224

How do you count subarrays whose sum equals k?

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

01234

Index starts at 0. Scan once for max — O(n).

Keep a prefix-sum frequency map starting with 0:1.

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.

Before you speak the answer, walk the interviewer through these steps:

1
Keep a prefix-sum frequency
map starting with 0:1.
2
For each prefix p,
add the count of p-k, then increment p.
3
Average O(n) time and
O(n) space.
4
Sliding window only works
when all numbers are positive, so hashing is the general interview solution.
5
Common mistake
What juniors usually get wrong.
6
Close
When you pick this over the alternative.

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

Say this line

“Average O(n) time and O(n) space.”

Break into beats

AverageOntimeandO

Speaking order

2987408337471632900

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

Keep a prefix-sum frequency map starting with 0:1. For each prefix p, add the count of p-k, then increment p.