Added solution in cpp Determine if a given linked list contains a cycle

[kavaljeetsingh-dev]

Solution in CPP of Linked List Cycle - Hacktoberfest2025
Problem Statement

LeetCode Problem: Linked List Cycle II
Determine the node where a cycle begins in a given linked list. If no cycle exists, return null.

💡 Approach
Cycle Detection (Floyd’s Cycle Detection Algorithm):
Use two pointers:
slow moves one step at a time.
fast moves two steps at a time.
If they ever meet, a cycle exists.
Finding the Start of the Cycle:
When slow and fast meet, move slow back to head.
Move both pointers one step at a time.
The node where they meet again is the start of the cycle.
If no cycle exists:
The fast pointer will reach nullptr, and we return nullptr.

🧠 Complexity Analysis
Time Complexity: O(N)
Each node is visited at most twice (once by slow, once by fast).
Space Complexity: O(1)
No extra data structures are used, only two pointers.

Example
Input:
head = [3,2,0,-4], pos = 1
Output:
Cycle detected at node with value: 2

[github-actions]

🧩 PR Checklist

Hi @kavaljeetsingh-dev, before merging please ensure:

• Updated contributor.md
• Added proper commit message
• Verified build/test success ✅

[github-actions]

⚠️ @kavaljeetsingh-dev, please add your details to the contributor.md file before merging. 🙏

[github-actions]

⭐ Hey @kavaljeetsingh-dev, if you like this project, don't forget to star the repository! 🚀

[ayushHardeniya]

@kavaljeetsingh-dev , kindly describe your approach and the complexities of the source code, in the PR description!

[ayushHardeniya]

Also, do remove the .vscode folder & files from the Pull Request, and recommit with the changes.

[kavaljeetsingh-dev]

changes done