LeetCode Linked List Cycle II Problem #139
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📝 WalkthroughWalkthroughThe PR reorganizes the Linked List Cycle II solution from a root-level directory into a standardized location under Changes
Estimated code review effort🎯 3 (Moderate) | ⏱️ ~20 minutes Poem
Pre-merge checks and finishing touches✅ Passed checks (3 passed)
✨ Finishing touches
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Claude finished @pertrai1's task in 1m 11s —— View job Code Review Complete ✅Excellent work on implementing Floyd's Cycle Detection algorithm! This is a classic two-pointer problem with a beautiful mathematical insight. Here's my detailed review: ✅ Algorithm CorrectnessYour solution is correct and handles all the required cases:
The logic is sound: first detect if a cycle exists, then reset one pointer to head and move both at the same pace to find the cycle start. 📊 Time & Space Complexity AnalysisYour complexity analysis is accurate:
This meets the follow-up challenge of using constant memory! Why O(n) time?
🎯 DSA Pattern: Floyd's Tortoise and Hare (Two Pointers)Pattern Name: Floyd's Cycle Detection Algorithm (Two Pointers - Fast & Slow) Why This Pattern?
Floyd's algorithm elegantly solves both with mathematical precision! 💡 Key Insight - The "Aha!" MomentThe non-obvious insight that makes this work:
Mathematical Proof:
When they meet:
Since fast moves 2x speed: Simplifying: This means: distance from head to start = distance from meeting point to start (mod C)! That's why resetting slow to head and moving both at the same pace works! 🔍 Code Quality ReviewStrengths:
Minor Suggestions:1. Comment Enhancement (line 23-24) Your comment "if they meet we are now ready to start figuring out the cycle" could explain why the algorithm works: // If they meet, a cycle exists. Mathematical insight: the distance from
// head to cycle start equals the distance from meeting point to cycle start.
// So reset slow to head and move both at same pace to find cycle start.
if (slow === fast) {2. Edge Case Documentation (line 14) Consider adding a comment about the empty list case: /**
* Detects the starting node of a cycle in a linked list using Floyd's algorithm
* @time - O(n) where n is the number of nodes in the list
* @space - O(1) because slow and fast are only pointers
* @returns The cycle start node, or null if no cycle or empty list
*/🔗 Related Problems (Same Pattern)Practice Floyd's algorithm with these:
🎓 Learning SummaryCore Concepts Mastered:
Common Pitfalls (You Avoided!):
Pattern Recognition:When you see:
📝 Post-Mortem ReviewYour POST_MORTEM_TEMPLATE.md is excellent! You correctly identified:
Suggestion for next time: In the post-mortem, consider adding the mathematical proof of why the algorithm works. It deepens understanding and helps with similar problems. 🎖️ Overall AssessmentCorrectness: ⭐⭐⭐⭐⭐ Flawless implementation Great job implementing this challenging algorithm! Your post-mortem shows strong metacognitive awareness. The fact that you identified and fixed the pointer reset issue demonstrates excellent debugging skills. | Branch: |
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🧮 Solution AnalysisPR: LeetCode Linked List Cycle II Problem 📊 Code Quality ResultsESLint Analysis🔍 Complexity Analysis💡 Analysis Tips
📁 Analysis Files CreatedDetailed analysis files have been automatically generated and committed to 🎯 Next Steps
Analysis generated for PR files only |
✅ Test ResultsTests have been executed for this pull request. 📊 Coverage Summary
Check the workflow logs for detailed test output. |
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🤖 Gemini Code Review (Model: Here's a review of the provided solution for LeetCode problem 142, "Linked List Cycle II," using the established review structure: 1. Pattern IdentificationThis solution uses the Floyd's Tortoise and Hare algorithm, also known as the Fast and Slow Pointers pattern. This pattern is primarily used for:
2. Complexity Verification
3. Key InsightThe key insight, or "aha!" moment, for solving "Linked List Cycle II" using Floyd's Tortoise and Hare algorithm is twofold:
4. Edge Case AnalysisThe current solution handles most edge cases correctly:
The solution appears robust for all standard edge cases related to linked list cycles. 5. Learning Points
6. Code Quality
7. Alternative Approaches
The chosen Floyd's Tortoise and Hare algorithm is the optimal and preferred solution due to its |
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🤖 Gemini Code Review (Model: This is a well-implemented solution for a classic linked list problem! It correctly utilizes the optimal algorithm. Let's break down the review. 1. Pattern IdentificationThis solution uses the Floyd's Tortoise and Hare algorithm, which is a specialized application of the Two Pointers pattern. Specifically, it uses two pointers moving at different speeds to first detect a cycle, and then to locate the starting node of that cycle. 2. Complexity Verification
3. Key InsightThe key insight for solving "Linked List Cycle II" using O(1) space, after detecting a cycle with the Tortoise and Hare algorithm, lies in a mathematical property derived from their movement:
4. Edge Case AnalysisThe current solution handles the following edge cases correctly:
All common edge cases seem to be handled correctly by the logic. 5. Learning Points
6. Code QualityThe code is clean, well-structured, and follows good practices.
// Original
slow = slow!.next;
fast = fast.next.next; // fast.next is already checked by while condition, but fast.next.next might be null if fast.next is the last node before fast becomes null
// Updated, if fast.next.next is potentially null, then fast.next.next should be handled gracefully.
// However, the `while (fast && fast.next)` condition ensures fast.next is not null.
// If fast.next is a valid node, fast.next.next could still be null if fast.next is the tail.
// Let's re-verify the original code's safety:
// `while (fast && fast.next)`:
// If fast is X and fast.next is Y:
// slow = slow.next (safe)
// fast = fast.next.next (safe to access fast.next, then safe to access its .next)
// If fast.next is the tail and its .next is null, then fast becomes null, and the loop terminates.
// The original code is correct and safe without the `!`.
// Proposed change for cleaner code:
export function detectCycle(head: ListNode | null): ListNode | null {
let slow = head;
let fast = head;
while (fast && fast.next) { // This condition ensures fast and fast.next are not null
slow = slow!.next; // Here slow is guaranteed to be a ListNode, so slow.next is safe
fast = fast.next.next; // Here fast.next is guaranteed to be a ListNode, so fast.next.next is safe to access
if (slow === fast) {
slow = head;
while (slow !== fast) {
slow = slow!.next; // Here slow is guaranteed to be a ListNode, so slow.next is safe
fast = fast!.next; // Here fast is guaranteed to be a ListNode, so fast.next is safe
}
return slow;
}
}
return null;
}Correction: My initial thought process for 7. Alternative Approaches
This solution is generally preferred due to its optimal space complexity. |
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Pull request overview
This PR adds a solution for LeetCode problem #142 "Linked List Cycle II" (Medium difficulty) and reorganizes the solution files from the root directory into the proper leetcode/medium/ directory structure. The solution implements Floyd's Tortoise and Hare algorithm to detect cycles in O(n) time with O(1) space complexity.
Key Changes
- Implements Floyd's cycle detection algorithm with proper TypeScript types and exports
- Adds comprehensive problem documentation and post-mortem analysis
- Updates repository problem counts (167→168 total, 73→74 medium problems)
Reviewed changes
Copilot reviewed 6 out of 6 changed files in this pull request and generated no comments.
Show a summary per file
| File | Description |
|---|---|
| leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts | New TypeScript solution implementing Floyd's Tortoise and Hare algorithm with ListNode class definition and detectCycle function |
| leetcode/medium/0142-linked-list-cycle-ii/README.md | Problem description with examples, constraints, and follow-up question from LeetCode |
| leetcode/medium/0142-linked-list-cycle-ii/POST_MORTEM_TEMPLATE.md | Detailed post-mortem documentation covering solution approach, pattern recognition, and learning takeaways |
| README.md | Updated problem counts and added link to new problem in the Medium Linked Lists section |
| 0142-linked-list-cycle-ii/linked-list-cycle-ii.ts | Removed file (relocated to proper directory structure) |
| 0142-linked-list-cycle-ii/README.md | Removed file (relocated to proper directory structure) |
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Actionable comments posted: 0
🧹 Nitpick comments (1)
leetcode/medium/0142-linked-list-cycle-ii/POST_MORTEM_TEMPLATE.md (1)
1-113: Post-mortem template is comprehensive and well-organized. ✓The template captures the complete problem-solving retrospective with sections for time tracking, solution exploration, pattern recognition, edge case analysis, mistake documentation, and self-assessment. The included content demonstrates genuine reflection (e.g., documenting the pointer-reset mistake at line 85) and knowledge consolidation (e.g., related problems at lines 59–60, cheat sheet entry at line 103).
However, consider whether problem-specific content (dates, links, concrete examples) should be moved to a generic template file for future reuse, with this instance as an example. The current approach is fine for one-off documentation but may limit applicability across the repository.
📜 Review details
Configuration used: Organization UI
Review profile: CHILL
Plan: Pro
📒 Files selected for processing (6)
0142-linked-list-cycle-ii/README.md0142-linked-list-cycle-ii/linked-list-cycle-ii.tsREADME.mdleetcode/medium/0142-linked-list-cycle-ii/POST_MORTEM_TEMPLATE.mdleetcode/medium/0142-linked-list-cycle-ii/README.mdleetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
💤 Files with no reviewable changes (2)
- 0142-linked-list-cycle-ii/README.md
- 0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
🧰 Additional context used
📓 Path-based instructions (12)
leetcode/**/README.md
📄 CodeRabbit inference engine (.github/copilot-instructions.md)
Problem README files should include: problem title with difficulty badge, problem description, examples with inputs and outputs, and constraints
Keep README files up to date with problem counts and organization
Files:
leetcode/medium/0142-linked-list-cycle-ii/README.md
leetcode/**
📄 CodeRabbit inference engine (.github/copilot-instructions.md)
Don't require tests for all LeetCode solutions (validated on platform)
Files:
leetcode/medium/0142-linked-list-cycle-ii/README.mdleetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.tsleetcode/medium/0142-linked-list-cycle-ii/POST_MORTEM_TEMPLATE.md
**/README.md
📄 CodeRabbit inference engine (CLAUDE.md)
Include problem title and difficulty badge, link to the problem on the respective platform (LeetCode, GreatFrontEnd, etc.), problem description, examples and constraints
Files:
leetcode/medium/0142-linked-list-cycle-ii/README.mdREADME.md
**/*.ts
📄 CodeRabbit inference engine (.github/copilot-instructions.md)
**/*.ts: Use explicit type annotations for function parameters and return types
Follow strict TypeScript configuration (defined intsconfig.json)
Target ES2020 with CommonJS modules
Strict mode enabled with ES2020 target and CommonJS modules for TypeScript
**/*.ts: Use explicit type annotations for function parameters and return types in TypeScript solutions
Follow strict TypeScript configuration with ES2020 target and CommonJS modules
Avoidanytype unless absolutely necessary in TypeScript solutions
Use interfaces for object types in TypeScript solutions
Files:
leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
**/*.{ts,js}
📄 CodeRabbit inference engine (.github/copilot-instructions.md)
**/*.{ts,js}: Test complex algorithms with helper functions, data structure implementations, solutions with multiple edge cases, GreatFrontEnd API implementations, and reusable utility functions
Use descriptive variable names (single letters likei,j,kare acceptable for loop iterators and standard mathematical contexts)
Don't use deprecated JavaScript features
Files:
leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
**/*-*.{ts,js,md}
📄 CodeRabbit inference engine (.github/copilot-instructions.md)
Use kebab-case for file names matching problem names
Files:
leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
leetcode/**/*.{ts,js}
📄 CodeRabbit inference engine (.github/copilot-instructions.md)
Don't deviate from LeetCode's expected function signatures
Files:
leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
**/*.{js,ts}
📄 CodeRabbit inference engine (CLAUDE.md)
**/*.{js,ts}: Verify the solution correctly solves the problem for all test cases, including edge cases: empty arrays, single elements, negative numbers, null values
Document and verify time and space complexity using Big O notation, typically in code comments or docs/analysis/ files
Use clear, descriptive variable names (not just i, j, k unless in simple loops) and add comments for non-obvious logic or algorithm steps
Use const by default, let only when reassignment is needed; avoid var except for LeetCode solution function definitions
Use strict equality (===) over loose equality (==)
Use modern ES6+ syntax (arrow functions, destructuring, spread operators) where appropriate and prefer built-in methods (.map(), .filter(), .reduce())
**/*.{js,ts}: Useconstby default,letonly when reassignment is needed in JavaScript/TypeScript
Use strict equality (===) over loose equality (==) in JavaScript/TypeScript
Use descriptive variable names; single letters likei,j,kacceptable only for standard loop contexts
Prefer built-in array methods (.map(),.filter(),.reduce()) in JavaScript/TypeScript
Fix ESLint errors withnpm run quality:fixand ensure proper code formatting with Prettier
Files:
leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
leetcode/**/*.{js,ts}
📄 CodeRabbit inference engine (AGENTS.md)
leetcode/**/*.{js,ts}: Keep LeetCode solutions concise and focused on the algorithm
Always document both time and space complexity in Big O notation with clear reasoning
Explain WHY the complexity is what it is, including amortized analysis when applicable
Document the algorithmic pattern used in solution code comments
Include examples in comments for complex algorithms in solution files
Review complexity analysis reports for algorithm files after running quality checks
Test edge cases if adding or modifying solution code
Do not add unnecessary dependencies or imports to solutions
Do not over-engineer simple solutions
Do not ignore time/space complexity considerations in code reviews
Do not deviate from LeetCode's expected function signatures in solution files
Files:
leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
leetcode/**/*-*.{js,ts}
📄 CodeRabbit inference engine (AGENTS.md)
Solution files should use kebab-case naming:
{problem-name}.{js|ts}
Files:
leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
leetcode/**/*.ts
📄 CodeRabbit inference engine (AGENTS.md)
Use TypeScript type annotations for all parameters and return types with complexity analysis in code comments
Files:
leetcode/medium/0142-linked-list-cycle-ii/linked-list-cycle-ii.ts
README.md
📄 CodeRabbit inference engine (AGENTS.md)
Update README.md when adding new solutions
Files:
README.md
🧠 Learnings (9)
📓 Common learnings
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-30T13:41:58.608Z
Learning: Applies to leetcode/**/README.md : Keep README files up to date with problem counts and organization
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-30T13:41:58.608Z
Learning: Applies to leetcode/*/*/ : Ensure problems are in correct difficulty folders (easy, medium, hard) in the LeetCode directory
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-12-30T13:41:07.646Z
Learning: Applies to leetcode/**/README.md : Problem README files should include: problem title with difficulty badge, problem description, examples with inputs and outputs, and constraints
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-30T13:41:58.608Z
Learning: Applies to leetcode/**/[0-9][0-9][0-9][0-9]-* : Problem solution folders should follow naming convention: `{difficulty}/{4-digit-number}-{problem-name}` in kebab-case
📚 Learning: 2025-12-30T13:41:58.608Z
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-30T13:41:58.608Z
Learning: Applies to leetcode/**/README.md : Keep README files up to date with problem counts and organization
Applied to files:
leetcode/medium/0142-linked-list-cycle-ii/README.mdREADME.md
📚 Learning: 2025-12-30T13:41:07.646Z
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-12-30T13:41:07.646Z
Learning: Applies to leetcode/**/README.md : Problem README files should include: problem title with difficulty badge, problem description, examples with inputs and outputs, and constraints
Applied to files:
leetcode/medium/0142-linked-list-cycle-ii/README.md
📚 Learning: 2025-12-30T13:41:58.608Z
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-30T13:41:58.608Z
Learning: Applies to leetcode/**/*.js : Add JSDoc comments explaining key logic and decisions in JavaScript solution files
Applied to files:
leetcode/medium/0142-linked-list-cycle-ii/README.md
📚 Learning: 2025-12-30T13:41:24.633Z
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: CLAUDE.md:0-0
Timestamp: 2025-12-30T13:41:24.633Z
Learning: Applies to **/README.md : Include problem title and difficulty badge, link to the problem on the respective platform (LeetCode, GreatFrontEnd, etc.), problem description, examples and constraints
Applied to files:
README.md
📚 Learning: 2025-12-30T13:41:58.608Z
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-30T13:41:58.608Z
Learning: Do not require tests for all LeetCode solutions (validated on platform)
Applied to files:
README.md
📚 Learning: 2025-12-30T13:41:07.646Z
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: .github/copilot-instructions.md:0-0
Timestamp: 2025-12-30T13:41:07.646Z
Learning: Applies to leetcode/** : Don't require tests for all LeetCode solutions (validated on platform)
Applied to files:
README.md
📚 Learning: 2025-12-30T13:41:58.608Z
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-30T13:41:58.608Z
Learning: Applies to leetcode/*/*/ : Ensure problems are in correct difficulty folders (easy, medium, hard) in the LeetCode directory
Applied to files:
README.md
📚 Learning: 2025-12-30T13:41:58.608Z
Learnt from: CR
Repo: pertrai1/coding-challenges PR: 0
File: AGENTS.md:0-0
Timestamp: 2025-12-30T13:41:58.608Z
Learning: Applies to docs/**/*analysis.md : Add badges for difficulty levels in problem documentation: ``
Applied to files:
README.md
🪛 LanguageTool
leetcode/medium/0142-linked-list-cycle-ii/README.md
[grammar] ~45-~45: Use a hyphen to join words.
Context: ...d index** in the linked-list. ## Follow up Can you solve it using O(1) (i.e. ...
(QB_NEW_EN_HYPHEN)
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- GitHub Check: review
🔇 Additional comments (2)
leetcode/medium/0142-linked-list-cycle-ii/README.md (1)
1-47: Documentation is well-structured and complete. ✓The README includes the problem title with difficulty badge, clear problem description, three illustrative examples with visual diagrams and explanations, comprehensive constraints, and a follow-up question to encourage optimization thinking. This fully aligns with the documentation guidelines and provides learners with a solid reference.
README.md (1)
9-9: Problem entry and counts updated correctly. ✓The LeetCode solved count (168), Medium count (74), section header, and new problem entry under Linked List are all properly synchronized and consistently formatted. The problem entry follows the established pattern with proper link path and difficulty badge.
Also applies to: 21-21, 185-185, 217-217
📅 Spaced Repetition Reviews Scheduled!Great job solving #142 - Linked List Cycle Ii! 🎉 To help you retain this knowledge long-term, I've scheduled 5 review sessions using spaced repetition:
What to expect:
🧠 Why Spaced Repetition?Research shows that reviewing material at increasing intervals dramatically improves retention. This adaptive scheduling ensures optimal spacing based on when you actually complete each review! Check |
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