feat(algorithms, hash table): first unique character in string

[BrianLusina]

Describe your change:

First unique character in string

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Documentation change?

Checklist:

• I have read CONTRIBUTING.md.
• This pull request is all my own work -- I have not plagiarized.
• I know that pull requests will not be merged if they fail the automated tests.
• This PR only changes one algorithm file. To ease review, please open separate PRs for separate algorithms.
• All new Python files are placed inside an existing directory.
• All filenames are in all lowercase characters with no spaces or dashes.
• All functions and variable names follow Python naming conventions.
• All function parameters and return values are annotated with Python type hints.
• All functions have doctests that pass the automated testing.
• All new algorithms have a URL in its comments that points to Wikipedia or other similar explanation.
• If this pull request resolves one or more open issues then the commit message contains Fixes: #{$ISSUE_NO}.

Summary by CodeRabbit

Release Notes

• New Features

  • Added the "First Unique Character" algorithm, which identifies the index of the first non-repeating character in a string, or -1 if none exists.

• Documentation

  • Provided comprehensive problem documentation with descriptions, example cases, and constraints.

• Tests

  • Added test suite to verify the algorithm's correctness across various test cases.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This pull request introduces a new algorithm for the "First Unique Character" problem in the hash table category. It includes problem documentation, a working implementation using character frequency counting, comprehensive test coverage with parameterized test cases, and updates to the directory index.

Changes

Cohort / File(s)	Summary
Documentation DIRECTORY.md, algorithms/hash_table/first_unique_character/README.md	Added directory index entry and comprehensive README documenting the problem statement, constraints, and three worked examples
Implementation algorithms/hash_table/first_unique_character/__init__.py	New first_unique_character(s: str) -> int function using collections.Counter to identify character frequencies and return the index of the first non-repeating character, or -1 if none exist
Tests algorithms/hash_table/first_unique_character/test_first_unique_character.py	New test module with parameterized test cases covering multiple input scenarios using unittest and parameterized decorators

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~15 minutes

Poem

🐰 A new algorithm hops into place, finding unique characters with grace!
The Counter counts with speed so bright,
First unique found—the answer's right! 🎯

🚥 Pre-merge checks | ✅ 2 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: adding a 'first unique character in string' algorithm to the hash table category.
Description check	✅ Passed	The description includes all required sections and checklist items are mostly complete; however, the 'Description' section lacks detail about what the change does, and one checklist item about issue resolution is correctly left unchecked.

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✨ Finishing touches

• 📝 Generate docstrings

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[coderabbitai]

Actionable comments posted: 2

🤖 Fix all issues with AI agents

In `@algorithms/hash_table/first_unique_character/README.md`:
- Around line 1-35: Add a reference URL for the problem statement to the README
for "First Unique Character": update the top description block (the section
starting with the title "First Unique Character" and the one-line problem
statement) to include a source link (for example LeetCode problem URL) labeled
clearly as "Reference" or "Source" immediately after the description paragraph
so readers can follow to the original problem; ensure the link is a full URL and
use the same README header area where the problem is described.

In `@DIRECTORY.md`:
- Around line 169-170: The unordered list items for "First Unique Character" are
mis-indented causing markdownlint MD007; fix by aligning the two list lines to
the same nesting level as the surrounding "Hash Table" block—ensure the bullet
for "First Unique Character" and its nested link line use consistent spaces
(match the other top-level list item indentation in the Hash Table section) so
both the item label "First Unique Character" and the link "[Test First Unique
Character](https://github.com/BrianLusina/PythonSnips/blob/master/algorithms/hash_table/first_unique_character/test_first_unique_character.py)"
share the same indentation depth and the UL nesting is consistent.