refactor: TwoPointers

[alxkm]

• Added a null check with an exception to fail fast if input is null. And test for this.
• Made class public for usability.
• Used clearer variable names (left and right) that are more descriptive than i and j.
• Expanded Javadoc for parameters and exceptions.

• 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 code is formatted with clang-format -i --style=file path/to/your/file.java

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 74.64%. Comparing base (182118b) to head (b17567a).

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #6374   +/-   ##
=========================================
  Coverage     74.63%   74.64%           
- Complexity     5443     5445    +2     
=========================================
  Files           682      682           
  Lines         18998    19000    +2     
  Branches       3668     3668           
=========================================
+ Hits          14179    14182    +3     
  Misses         4260     4260           
+ Partials        559      558    -1     

☔ 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.