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This repository is a comprehensive academic + practical exploration of Queue data structures in Java, starting from basic theory to advanced problem-solving and transformations.
It includes:
- Multiple implementations (Array, Linked List, JCF)
- Advanced structures like Circular Queue & Deque
- Stack ↔ Queue transformations
- Real-world problem-solving patterns
A Queue is a linear data structure that follows:
FIFO (First In First Out)
- The element inserted first is removed first
- Similar to a line/queue in real life
| Operation | Description |
|---|---|
| Enqueue | Add element to rear |
| Dequeue | Remove element from front |
| Peek | Get front element |
| isEmpty | Check if queue is empty |
- Linear structure
- Inefficient in arrays due to shifting
- Solves space wastage problem
- Uses modulo arithmetic
(rear + 1) % size
- Insert/delete from both ends
- Can behave as:
- Queue
- Stack
(Not implemented in this module)
- Elements processed based on priority
- Fixed size
- Needs shifting → O(n)
- Dynamic size
- Efficient operations → O(1)
Efficient memory usage using wrap-around
Used in Queue using 2 Stacks
- Stack → Queue
- Queue → Stack
📄 Code:
- Uses a simple array
- Dequeue requires shifting elements
| Operation | Time |
|---|---|
| Enqueue | O(1) |
| Dequeue | O(n) ❌ (due to shifting) |
| Peek | O(1) |
| Space | O(n) |
📄 Code:
- Avoids shifting
- Uses front & rear pointers
- Wrap-around using modulo
| Operation | Time |
|---|---|
| Enqueue | O(1) |
| Dequeue | O(1) |
| Peek | O(1) |
| Space | O(n) |
📄 Code:
- Uses nodes (head & tail)
- Dynamic memory allocation
| Operation | Time |
|---|---|
| Enqueue | O(1) |
| Dequeue | O(1) |
| Peek | O(1) |
| Space | O(n) |
📄 Code:
- Uses LinkedList implementation of Queue
- Production-ready
| Operation | Time |
|---|---|
| Enqueue | O(1) |
| Dequeue | O(1) |
| Peek | O(1) |
📄 Code:
📄 Code:
- Supports both ends
- Acts as both:
- Queue
- Stack
All operations:
- O(1)
📄 Code:
-
Two stacks:
- s1 → input
- s2 → output
-
Lazy transfer
| Operation | Time |
|---|---|
| Enqueue | O(1) |
| Dequeue | Amortized O(1) |
| Peek | Amortized O(1) |
📄 Code:
- Uses addLast() & removeFirst()
- All operations → O(1)
📄 Code:
- Push is costly (reordering elements)
| Operation | Time |
|---|---|
| Push | O(n) |
| Pop | O(1) |
| Peek | O(1) |
📄 Code:
- Uses addLast() & removeLast()
- All operations → O(1)
📄 Code:
- Track frequency
- Maintain order using queue
- Time: O(n)
- Space: O(1) (fixed 26 array)
📄 Code:
- Split + merge alternately
- Time: O(n)
- Space: O(n)
📄 Code:
- Use stack as auxiliary
- Time: O(n)
- Space: O(n)
| Structure | Enqueue | Dequeue | Peek | Space |
|---|---|---|---|---|
| Array Queue | O(1) | O(n) | O(1) | O(n) |
| Circular Queue | O(1) | O(1) | O(1) | O(n) |
| Linked List Queue | O(1) | O(1) | O(1) | O(n) |
| JCF Queue | O(1) | O(1) | O(1) | O(n) |
| Queue via Stack | O(1) | Amortized O(1) | O(1) | O(n) |
| Stack via Queue | O(n) | O(1) | O(1) | O(n) |
| Deque | O(1) | O(1) | O(1) | O(n) |
javac FileName.java
java FileName- CPU Scheduling
- BFS (Graphs)
- Streaming systems
- Task queues (OS, servers)
- Message queues (Kafka, RabbitMQ concept)
✔ Circular Queue solves array inefficiency ✔ Deque is the most flexible structure ✔ Stack ↔ Queue conversion builds deep understanding ✔ JCF should be used in real-world applications
Satinder Singh Sall Full-Stack Web/Mobile Engineer 📘 Data Structures (Java) 🎯 Academic + Interview Preparation
This project is for educational and portfolio purposes
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