refactor: ReturnSubsequence

[alxkm]

Class cleanup.
Readability improvements.
Added tests.

• 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 49.12%. Comparing base (49d1c84) to head (2d40cc7).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5408      +/-   ##
============================================
+ Coverage     49.02%   49.12%   +0.09%     
- Complexity     3049     3052       +3     
============================================
  Files           524      524              
  Lines         15242    15231      -11     
  Branches       2899     2898       -1     
============================================
+ Hits           7473     7482       +9     
+ Misses         7461     7442      -19     
+ Partials        308      307       -1     

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