Refactor: Combination

[samuelfac]

Related to #5164

• 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.
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[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 2 lines in your changes missing coverage. Please review.

Project coverage is 40.32%. Comparing base (8ea90fd) to head (a25969d).

Files	Patch %	Lines
...va/com/thealgorithms/backtracking/Combination.java	83.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #5225      +/-   ##
============================================
- Coverage     40.35%   40.32%   -0.04%     
+ Complexity     2486     2484       -2     
============================================
  Files           519      519              
  Lines         15479    15474       -5     
  Branches       2950     2949       -1     
============================================
- Hits           6247     6240       -7     
- Misses         8943     8944       +1     
- Partials        289      290       +1     

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[vil02]

I do not see a reason to return null here. You can just remove this branch, so the result would be Arrays.asList(new TreeSet<Integer>()).

The problem is ArrayCombination. The method combination will need an update. I force merged #5181, so this null problem will no longer occur.

[vil02]

Could you please:

• add a test case with data being an empty array and combinationSize being positive,
• add a test case with data being an empty array and combinationSize being zero,
• add a test case with data being [1, 2, 3] and combinationSize being 2,
• add a test case with data being an array of strings.

[vil02]

This is not verified anywhere. Maybe the input type should be Set<T>? Or maybe this statement is not needed. I think what we are aiming for it to mimic the itertools.combinations from python.

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[github-actions]

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