Project 5: Linked List From Scratch

Implement singly and doubly linked lists with full operations and tests.

Quick Reference

Attribute Value
Difficulty Level 2: Intermediate
Time Estimate 1-2 weeks
Language C (Alternatives: Rust, Go, Python)
Prerequisites Pointers, malloc/free, basic structs
Key Topics Pointers, memory management, list operations

1. Learning Objectives

By completing this project, you will:

  1. Implement singly and doubly linked lists in C.
  2. Manage memory safely with malloc/free.
  3. Analyze time complexity of list operations.
  4. Apply two-pointer techniques for list problems.

2. Theoretical Foundation

2.1 Core Concepts

  • Node and pointer: Each node stores data and a pointer to the next node.
  • Head and tail: Keeping a tail pointer can reduce append to O(1).
  • Doubly linked list: Back pointers allow O(1) deletion with node reference.
  • Sentinel nodes: Simplify edge cases by avoiding null checks.

2.2 Why This Matters

Linked lists teach manual memory management and pointer discipline. They underpin stacks, queues, hash table chaining, and LRU caches.

2.3 Historical Context / Background

Linked structures are a classic data structure from early computer science, designed to avoid array resizing costs.

2.4 Common Misconceptions

  • Misconception: Linked lists are always better than arrays. Reality: Cache locality often makes arrays faster.
  • Misconception: Deleting a node is always O(1). Reality: Finding the node is often O(n).

3. Project Specification

3.1 What You Will Build

A reusable linked list library with insert, delete, search, reverse, and cycle detection, plus a test suite.

3.2 Functional Requirements

  1. Singly list: push, pop, insert, delete, search.
  2. Doubly list: forward/back traversal, insert at both ends.
  3. Utilities: reverse, find middle, detect cycle.
  4. CLI demo: interactive list operations.

3.3 Non-Functional Requirements

  • Reliability: No memory leaks; valgrind clean.
  • Usability: Clear API with header files.
  • Performance: O(1) head insert, O(1) tail append if tail used.

3.4 Example Usage / Output

$ ./list-demo
> push 3
> push 2
> push 1
List: 1 -> 2 -> 3
> reverse
List: 3 -> 2 -> 1

3.5 Real World Outcome

You can build and run the CLI to insert and remove nodes. Memory checks should show no leaks, and operations like reverse and cycle detection should work on arbitrary lists.

4. Solution Architecture

4.1 High-Level Design

┌──────────────┐     ┌──────────────┐     ┌──────────────┐
│ CLI / Tests  │────▶│ List Library │────▶│ Memory Tools │
└──────────────┘     └──────────────┘     └──────────────┘

4.2 Key Components

Component Responsibility Key Decisions
list.h Public API Opaque struct or direct node exposure
list.c Implementation Sentinel nodes for simplicity
tests Correctness Use small fixed sequences

4.3 Data Structures

typedef struct Node {
    int value;
    struct Node* next;
    struct Node* prev; // optional for doubly
} Node;

4.4 Algorithm Overview

Key Algorithm: Cycle Detection (Floyd)

  1. Move slow pointer by 1, fast by 2.
  2. If they meet, a cycle exists.

Complexity Analysis:

  • Time: O(n)
  • Space: O(1)

5. Implementation Guide

5.1 Development Environment Setup

cc -Wall -Wextra -o list-demo src/*.c
valgrind ./list-demo

5.2 Project Structure

linked-list/
├── src/
│   ├── list.c
│   ├── list.h
│   ├── demo.c
├── tests/
└── README.md

5.3 The Core Question You’re Answering

“How does pointer-based data storage actually work in memory?”

5.4 Concepts You Must Understand First

Stop and research these before coding:

  1. Pointers and memory addresses
  2. malloc/free lifecycle
  3. Struct layout in C

5.5 Questions to Guide Your Design

  1. Will you expose Node or keep it private?
  2. How will you handle empty list edge cases?
  3. How will you make append O(1)?

5.6 Thinking Exercise

Draw a list of three nodes and show the pointers for next and prev. Then remove the middle node and update pointers.

5.7 The Interview Questions They’ll Ask

  1. Detect a cycle in a linked list.
  2. Reverse a linked list iteratively.
  3. Find the middle node in one pass.

5.8 Hints in Layers

Hint 1: Start with head-only list Add tail optimization after basic operations work.

Hint 2: Use a sentinel A dummy head avoids null checks.

Hint 3: Validate with valgrind Check for leaks after each operation.

5.9 Books That Will Help

Topic Book Chapter
Pointers “C Programming: A Modern Approach” Ch. 11
Linked lists “Mastering Algorithms with C” Ch. 5

5.10 Implementation Phases

Phase 1: Foundation (3-4 days)

Goals:

  • Singly linked list operations
  • Basic tests

Tasks:

  1. Implement push, pop, insert, delete.
  2. Write tests for small lists.

Checkpoint: All basic ops pass tests.

Phase 2: Doubly Linked List (3-4 days)

Goals:

  • Add prev pointers
  • Implement delete with node reference

Tasks:

  1. Add prev field.
  2. Implement forward/back traversal.

Checkpoint: Doubly list operations pass tests.

Phase 3: Algorithms (2-3 days)

Goals:

  • Reverse, cycle detection, middle
  • CLI demo

Tasks:

  1. Implement reverse and cycle detection.
  2. Add CLI commands.

Checkpoint: Demo supports key operations.

5.11 Key Implementation Decisions

Decision Options Recommendation Rationale
Tail pointer none, keep tail keep tail O(1) append
Sentinel node yes/no yes Fewer edge cases
Node exposure public/private private Safer API

6. Testing Strategy

6.1 Test Categories

Category Purpose Examples
Unit Tests Node ops insert, delete
Integration Tests CLI workflows push, reverse, print
Edge Case Tests Empty list delete on empty

6.2 Critical Test Cases

  1. Delete head, middle, tail.
  2. Reverse a single-node list.
  3. Detect cycle correctly.

6.3 Test Data

[]
[1]
[1,2,3,4]

7. Common Pitfalls and Debugging

7.1 Frequent Mistakes

Pitfall Symptom Solution
Double free Crash Set pointers to NULL after free
Lost nodes Memory leaks Update pointers carefully
Off-by-one in insertion Wrong order Track prev and current

7.2 Debugging Strategies

  • Print addresses to validate links.
  • Use valgrind after each change.

7.3 Performance Traps

Traversing from head to tail for every append yields O(n^2) construction. Use a tail pointer.

8. Extensions and Challenges

8.1 Beginner Extensions

  • Add list length tracking.
  • Add a “find by value” function.

8.2 Intermediate Extensions

  • Implement a sorted list insert.
  • Add an iterator interface.

8.3 Advanced Extensions

  • Implement a skip list variant.
  • Build an LRU cache on top of the list.

9. Real-World Connections

9.1 Industry Applications

  • LRU caches (list + hash table)
  • Memory allocators and free lists
  • glib: C data structures including lists.
  • Redis: Uses lists for data structures.

9.3 Interview Relevance

Linked list problems appear frequently and test pointer reasoning.

10. Resources

10.1 Essential Reading

  • “C Programming: A Modern Approach” by K.N. King - Ch. 11
  • “Mastering Algorithms with C” by Kyle Loudon - Ch. 5

10.2 Video Resources

  • Linked list visualizations (search: “linked list pointer animation”)

10.3 Tools and Documentation

  • Valgrind: https://valgrind.org

11. Self-Assessment Checklist

11.1 Understanding

  • I can draw a list in memory.
  • I can explain why insert at head is O(1).
  • I can detect a cycle with two pointers.

11.2 Implementation

  • No memory leaks in valgrind.
  • All operations pass tests.

11.3 Growth

  • I can reason about pointer updates without confusion.

12. Submission / Completion Criteria

Minimum Viable Completion:

  • Singly linked list with insert/delete/search
  • Basic tests and demo

Full Completion:

  • Doubly linked list
  • Reverse and cycle detection

Excellence (Going Above and Beyond):

  • LRU cache or skip list extension
  • Extensive test coverage

This guide was generated from LEARN_ALGORITHMS_DEEP_DIVE.md. For the complete learning path, see the parent directory.