refactor: `LongestNonRepetitiveSubstring`

[alxkm]

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Class cleanup. Readability and efficiency improvements. Added tests.

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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 88.88889% with 1 line in your changes missing coverage. Please review.

Please upload report for BASE (master@c413f3c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...orithms/strings/LongestNonRepetitiveSubstring.java	88.88%	0 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5421   +/-   ##
=========================================
  Coverage          ?   49.71%           
  Complexity        ?     3084           
=========================================
  Files             ?      524           
  Lines             ?    15171           
  Branches          ?     2884           
=========================================
  Hits              ?     7542           
  Misses            ?     7314           
  Partials          ?      315           

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