Refactor: simplify validation and improve backtracking cleanup

[swativ15]

This PR makes small readability and maintainability improvements to the algorithm implementation.

Changes

• Removed a redundant n < 0 validation check since the method contract already ensures valid n
• Replaced current.remove(current.size() - 1) with current.removeLast() to better express backtracking intent

Rationale

• Simplifies input validation without changing behavior
• Uses the Deque API to make the backtracking step clearer and less error-prone

Impact

• No change in algorithm logic or time/space complexity
• Output remains identical

• 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 algorithms include a corresponding test class that validates their functionality.
• 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 79.31%. Comparing base (249b88f) to head (a576673).

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7258      +/-   ##
============================================
- Coverage     79.31%   79.31%   -0.01%     
+ Complexity     6989     6988       -1     
============================================
  Files           781      781              
  Lines         22955    22955              
  Branches       4509     4509              
============================================
- Hits          18207    18206       -1     
  Misses         4020     4020              
- Partials        728      729       +1     

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