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DSA Pattern

Hash Set Pattern

Recognise the Hash Set pattern once — then solve any problem that uses it. Below are every Hash Set problem we index, grouped by the module that teaches them.

Problems

Walkthroughs ready

3E · 3M · 0H

Difficulty mix

Java · Python

Languages

Where this pattern shows up

Hash Set problems by module

Arrays & Hashing3 problems

Weighted Uniform Strings

AmazonMicrosoftbloomberg

AmazonGooglebloomberg

easyExplained

Stack & Queue1 problem

Equal Stacks

AmazonMicrosoftgoldman-sachs

mediumExplained

Linked List2 problems

Find the Merge Point of Two Linked Lists

AmazonMicrosoftGoogle

mediumExplained

Detect a Cycle in a Linked List

AmazonMicrosoftGoogle

mediumExplained

Pattern guide

Mastering the Hash Set pattern

Overview

The Hash Set pattern is one of the reusable techniques that experienced candidates spot instantly. Rather than memorising individual problems, strong interviewees recognise that a new question "is just Hash Set" and reach for the matching template. That recognition is exactly what this collection is designed to build.

We index 6 problems that use the Hash Set pattern (3 easy, 3 medium, 0 hard), each worked brute-force → optimal in Java and Python with line-by-line explanations. Solving them back-to-back is the fastest way to internalise when and how to apply Hash Set.

Pattern-first practice compounds: once Hash Set is automatic, you'll see it inside problems that never mention it by name.

Study playbook

1
Solve them in a single sitting
Work several Hash Set problems in a row. The shared skeleton becomes obvious when you see the same structure solve different-looking questions.
2
Name the trigger
Write down the signal that should make you reach for Hash Set (e.g. 'contiguous sub-range', 'sorted + pair', 'k largest'). In the interview, that trigger is your cue.
3
Keep one template
Maintain a single mental template for Hash Set that you can write from memory and adapt, instead of re-deriving it under pressure.
4
Compare with neighbours
Know how Hash Set differs from adjacent patterns so you pick the right tool when a problem could go more than one way.

Common pitfalls

Forcing Hash Set onto a problem that doesn't fit because it's the pattern you just practised.
Recalling a specific solution instead of re-deriving from the pattern when given a variation.
Skipping the brute force and the complexity discussion that frame your optimal answer.
Missing the edge cases the pattern is notorious for (empty input, boundaries, duplicates).

Frequently asked

What is the Hash Set pattern?

It's a reusable problem-solving technique that recurs across many interview questions. Learning it once lets you solve a whole family of problems instead of memorising each one.

When should I use Hash Set?

When a problem shows the pattern's trigger signal. The problems below all share that signal, so working through them trains you to recognise it quickly.

How many Hash Set problems should I do?

Enough that the template feels automatic — usually solving the indexed set here in sequence is sufficient to recognise Hash Set on sight.

Related patterns

Hash Map50

Keep exploring

Browse DSA another way

Curriculum

18 modules · theory + practice

Explore

Curated sheets

Blind 75, NeetCode 150, Grind 75

DSA Pattern

Hash Set Pattern

Recognise the Hash Set pattern once — then solve any problem that uses it. Below are every Hash Set problem we index, grouped by the module that teaches them.

Problems

Walkthroughs ready

3E · 3M · 0H

Difficulty mix

Java · Python

Languages

Where this pattern shows up

Hash Set problems by module

Arrays & Hashing3 problems

Weighted Uniform Strings

AmazonMicrosoftbloomberg

AmazonGooglebloomberg

easyExplained

Stack & Queue1 problem

Equal Stacks

AmazonMicrosoftgoldman-sachs

mediumExplained

Linked List2 problems

Find the Merge Point of Two Linked Lists

AmazonMicrosoftGoogle

mediumExplained

Detect a Cycle in a Linked List

AmazonMicrosoftGoogle

mediumExplained

Pattern guide

Mastering the Hash Set pattern

Overview

Pattern-first practice compounds: once Hash Set is automatic, you'll see it inside problems that never mention it by name.

Study playbook

1
Solve them in a single sitting
Work several Hash Set problems in a row. The shared skeleton becomes obvious when you see the same structure solve different-looking questions.
2
Name the trigger
Write down the signal that should make you reach for Hash Set (e.g. 'contiguous sub-range', 'sorted + pair', 'k largest'). In the interview, that trigger is your cue.
3
Keep one template
Maintain a single mental template for Hash Set that you can write from memory and adapt, instead of re-deriving it under pressure.
4
Compare with neighbours
Know how Hash Set differs from adjacent patterns so you pick the right tool when a problem could go more than one way.

Common pitfalls

Forcing Hash Set onto a problem that doesn't fit because it's the pattern you just practised.
Recalling a specific solution instead of re-deriving from the pattern when given a variation.
Skipping the brute force and the complexity discussion that frame your optimal answer.
Missing the edge cases the pattern is notorious for (empty input, boundaries, duplicates).

Frequently asked

What is the Hash Set pattern?

It's a reusable problem-solving technique that recurs across many interview questions. Learning it once lets you solve a whole family of problems instead of memorising each one.

When should I use Hash Set?

When a problem shows the pattern's trigger signal. The problems below all share that signal, so working through them trains you to recognise it quickly.

How many Hash Set problems should I do?

Enough that the template feels automatic — usually solving the indexed set here in sequence is sufficient to recognise Hash Set on sight.