Add ReturnOnInvestment to maths

[InukaWijerathna]

Description

Adds ReturnOnInvestment to the maths package.

Return on Investment (ROI) is a fundamental financial metric that quantifies how profitable an investment is relative to its cost:

ROI = (Gain from Investment - Cost of Investment) / Cost of Investment × 100

What's included

• ReturnOnInvestment.java — clean implementation with Javadoc and IllegalArgumentException for invalid cost
• ReturnOnInvestmentTest.java — 6 JUnit 5 tests covering positive, zero, and negative ROI, plus error paths

Checklist

• Follows the code style of the project (final class, private constructor, static method)
• JUnit 5 tests for all branches including exception cases
• Javadoc with formula and reference link

Reference: https://www.investopedia.com/terms/r/returnoninvestment.asp

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.72%. Comparing base (cecfad9) to head (2bced69).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #7446      +/-   ##
============================================
- Coverage     79.72%   79.72%   -0.01%     
- Complexity     7298     7300       +2     
============================================
  Files           803      804       +1     
  Lines         23762    23771       +9     
  Branches       4674     4675       +1     
============================================
+ Hits          18945    18952       +7     
  Misses         4059     4059              
- Partials        758      760       +2     

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[DenizAltunkapan]

Thanks for the contribution. The implementation itself is correct and the tests cover the basic cases well.

However, I am not fully convinced that this belongs in TheAlgorithms/Java as an algorithm. This is essentially a direct one-line financial formula rather than an algorithmic implementation. Accepting this may also set a precedent for many similar financial-ratio helper methods.

I would not approve this as-is...

[InukaWijerathna]

Thanks for the feedback! I've expanded the implementation to address the concern. The class now includes annualizedReturnOnInvestment(), which converts a total ROI over a multi-year holding period into an equivalent annual rate using the geometric-mean formula:

Annualized ROI = ((1 + ROI/100)^(1/n) - 1) x 100

This chains the simple ROI calculation, applies Math.pow with a fractional exponent, and validates an additional parameter (holding period in years). I've also added 8 new tests covering the one-year identity case, multi-year, fractional years, break-even, negative ROI, and invalid inputs. Happy to adjust further if needed!