refactor: MaximumSumOfDistinctSubarraysWithLengthK

[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 50.78%. Comparing base (cd38531) to head (b1f4568).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5433      +/-   ##
============================================
- Coverage     50.79%   50.78%   -0.01%     
  Complexity     3139     3139              
============================================
  Files           523      523              
  Lines         15109    15108       -1     
  Branches       2875     2874       -1     
============================================
- Hits           7674     7673       -1     
  Misses         7114     7114              
  Partials        321      321              

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