DSA Hash Sets
DSA Hash Sets – Complete Beginner Guide
(Concept • Operations • Examples • Implementation • Complexity)
A Hash Set is a hash-based data structure used to store unique elements only.
It’s extremely useful when you only need to check existence or remove duplicates—fast.
1️⃣ What is a Hash Set?
A Hash Set:
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Stores values only (no key–value pairs)
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Does not allow duplicates
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Uses a hash function internally
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Does not maintain order
Example
2️⃣ Why Use a Hash Set?
✔ Fast membership checks
✔ Automatic duplicate removal
✔ Clean logic for set-based problems
Best for:
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Checking if an element exists
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Removing duplicates
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Union / intersection problems
3️⃣ Hash Set vs Hash Table (Quick Compare)
| Feature | Hash Set | Hash Table |
|---|---|---|
| Stores | Values only | Key–Value pairs |
| Duplicates | ❌ Not allowed | ❌ Keys not allowed |
| Average Time | O(1) | O(1) |
| Use Case | Uniqueness | Mapping |
4️⃣ How Hash Set Works Internally
Internally, a Hash Set uses a hash table:
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Value → hash function
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Hash → index (bucket)
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Store value in that bucket
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Collisions handled via chaining or probing
5️⃣ Basic Operations
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add(x) → Insert if not present
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remove(x) → Delete if present
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contains(x) → Check existence
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size() → Number of unique elements
6️⃣ Hash Set Example (C++ – STL)
Using unordered_set:
Output (order may vary):
7️⃣ Remove Duplicates Using Hash Set
8️⃣ Hash Set – Basic Implementation Idea (Concept)
📌 Real implementations also resize the table to keep operations fast.
9️⃣ Time Complexity
| Operation | Average | Worst Case |
|---|---|---|
| Insert | O(1) | O(n) |
| Search | O(1) | O(n) |
| Delete | O(1) | O(n) |
📌 Worst case happens when many elements collide.
🔟 Space Complexity
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O(n) to store elements
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Extra space for buckets/collision handling
1️⃣1️⃣ Load Factor (Important)
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High load factor → more collisions
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Hash sets resize automatically to maintain performance
1️⃣2️⃣ Real DSA Uses
✔ Remove duplicates
✔ Check duplicates
✔ Find union / intersection
✔ Cycle detection
✔ Two-sum (existence check)
Interview FAQs
Q1. Does Hash Set allow duplicates?
👉 No
Q2. Is Hash Set ordered?
👉 No (unordered)
Q3. Why is Hash Set fast?
👉 Hash-based indexing (O(1) average)
Q4. When not to use Hash Set?
👉 When order matters or memory is tight
Summary
✔ Hash Set stores unique values
✔ Uses hashing internally
✔ Average O(1) operations
✔ Unordered, fast, and practical
✔ Very common in interviews & coding rounds
