Add Middle of Linked List (Slow/Fast Pointers)

[GziXnine]

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized it.
• All filenames are in PascalCase.
• All functions and variable names follow Java naming conventions.
• All new algorithms have a URL in their comments that points to Wikipedia or other similar explanations.
• All new algorithms include a corresponding test class that validates their functionality.
• All new code is formatted with clang-format -i --style=file path/to/your/file.java

Problem Clarification

The goal is to return the middle node of a singly linked list:

• For an odd-length list → return the exact middle node.
• For an even-length list → return the second middle node, as commonly defined in standard problem statements.

This behavior naturally falls out of the proposed algorithm without additional condition handling.

Algorithmic Approach

I will use the two-pointer (slow & fast) technique, which provides the optimal balance between clarity, correctness, and performance.

Methodology:

• Initialize two pointers at the head:
  • slow advances by one node
  • fast advances by two nodes
• Continue traversal while:

 fast != null && fast.next != null 

• When fast reaches the end of the list, slow will be positioned at the desired middle node.

This approach avoids unnecessary pre-computation (such as list length) and works in a single pass.

Complexity Guarantees

• Time Complexity: O(n) — single traversal
• Space Complexity: O(1) — constant extra space

This is asymptotically optimal for this problem.

Closes #6615

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.04%. Comparing base (babc762) to head (5768de2).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7212      +/-   ##
============================================
- Coverage     79.05%   79.04%   -0.02%     
- Complexity     6941     6943       +2     
============================================
  Files           778      779       +1     
  Lines         22898    22906       +8     
  Branches       4501     4502       +1     
============================================
+ Hits          18103    18106       +3     
- Misses         4076     4078       +2     
- Partials        719      722       +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[DenizAltunkapan]

@GziXnine Thanks