DSA Hash Tables
DSA Hash Tables – Complete Beginner Guide
(Concept • Hash Function • Collisions • Operations • Implementation • Complexity)
A Hash Table is one of the fastest data structures for storing and retrieving data.
It provides average O(1) time for search, insert, and delete.
1️⃣ What is a Hash Table?
A Hash Table stores data in key–value pairs.
Key → unique identifier
Value → data associated with the key
Hash Function → converts key into an index
📌 Data is stored in an array-like structure using the computed index.
2️⃣ Why Hash Tables Are Fast
Direct access using index
No need to traverse like arrays or linked lists
Ideal for dictionaries, caches, databases
3️⃣ Hash Function
A hash function:
Takes a key
Produces a fixed-size integer (index)
Distributes keys uniformly
Example
📌 Good hash functions reduce collisions.
4️⃣ What is a Collision?
A collision occurs when two different keys map to the same index.
5️⃣ Collision Resolution Techniques
1. Chaining
Each table index stores a linked list
All collided elements go into the list
✔ Simple
❌ Extra memory
2. Open Addressing
Store elements in the next available slot.
Types:
Linear Probing →
index + 1Quadratic Probing →
index + i²Double Hashing → second hash function
✔ No extra memory
❌ Clustering problem
6️⃣ Basic Hash Table Operations
Insert
Compute index using hash function
Store value at index (handle collision)
Search
Compute index
Check index (and linked list if chaining)
Delete
Find key
Remove value safely
7️⃣ Hash Table Using Chaining (C++ Example)
8️⃣ Hash Table Using Open Addressing (Linear Probing)
9️⃣ Time Complexity
| Operation | Average | Worst Case |
|---|---|---|
| Insert | O(1) | O(n) |
| Search | O(1) | O(n) |
| Delete | O(1) | O(n) |
📌 Worst case occurs when many collisions happen.
🔟 Space Complexity
O(n) for storing elements
Extra space for chaining (if used)
1️⃣1️⃣ Load Factor
📌 High load factor → more collisions
📌 Resize table when load factor increases
1️⃣2️⃣ Real-Life Uses of Hash Tables
✔ Dictionaries & Maps
✔ Database indexing
✔ Caching (LRU Cache)
✔ Symbol tables in compilers
✔ Password verification
1️⃣3️⃣ Hash Table vs Array
| Feature | Hash Table | Array |
|---|---|---|
| Access | O(1) avg | O(1) |
| Key type | Any | Integer index |
| Order | Unordered | Ordered |
| Flexibility | High | Low |
Interview Questions (Hash Tables)
Q1. What is a hash table?
👉 Key–value data structure with fast access
Q2. What is collision?
👉 Two keys map to same index
Q3. Best collision handling technique?
👉 Depends on use case
Q4. Why worst case is O(n)?
👉 Too many collisions
✅ Final Summary
✔ Hash Tables store key–value pairs
✔ Use hash functions for indexing
✔ Collisions handled by chaining or probing
✔ Average time complexity: O(1)
✔ Very important for DSA & interviews
