feat(algorithms, two-pointers): backspace string compare

[BrianLusina]

Describe your change:

Algorithms to check if two strings are equal after backspace characters
are applied to them.

• 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

  • Three solution approaches added for the Backspace String Compare algorithm, utilising two-pointer and string-building strategies.

• Documentation

  • Problem README added with problem definition, worked examples, constraints and complexity analysis for both solution methods.

• Tests

  • Parametrised unit tests added covering multiple backspace patterns to validate all three implementations.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: c57b7baa-b1bb-4db1-92c1-5c8b2fb60b5a

📥 Commits

Reviewing files that changed from the base of the PR and between 70bb453 and 10567e6.

⛔ Files ignored due to path filters (4)

• algorithms/two_pointers/backspace_string_compare/images/examples/backspace_string_compare_example_1.png is excluded by !**/*.png
• algorithms/two_pointers/backspace_string_compare/images/examples/backspace_string_compare_example_2.png is excluded by !**/*.png
• algorithms/two_pointers/backspace_string_compare/images/examples/backspace_string_compare_example_3.png is excluded by !**/*.png
• algorithms/two_pointers/backspace_string_compare/images/examples/backspace_string_compare_example_4.png is excluded by !**/*.png

📒 Files selected for processing (4)

• DIRECTORY.md
• algorithms/two_pointers/backspace_string_compare/README.md
• algorithms/two_pointers/backspace_string_compare/__init__.py
• algorithms/two_pointers/backspace_string_compare/test_backspace_string_compare.py

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📝 Walkthrough

Walkthrough

Adds a new backspace_string_compare module under algorithms/two_pointers with three implementations (backspace_compare_two_pointers, backspace_compare_two_pointers_2, backspace_compare_build_string), a parameterised unit test suite, a problem README, and a DIRECTORY.md entry.

Changes

Backspace String Compare

Layer / File(s)	Summary
Three solution functions algorithms/two_pointers/backspace_string_compare/__init__.py	Implements backspace_compare_two_pointers using a reversed generator with zip_longest, backspace_compare_two_pointers_2 using end-to-start index traversal with per-string skip counters, and backspace_compare_build_string using a list-as-stack simulation.
Parameterised unit tests algorithms/two_pointers/backspace_string_compare/test_backspace_string_compare.py	Defines shared (s, t, expected) test cases covering various backspace patterns and runs them against all three implementations via parameterized.expand.
README and directory index algorithms/two_pointers/backspace_string_compare/README.md, DIRECTORY.md	Adds the full problem README with statement, examples, constraints, and complexity analysis for both approaches, plus the DIRECTORY.md link entry under Two Pointers.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

Suggested labels

enhancement, Documentation

Poem

🐇 Tap tap tap, a # appears,
Erasing letters, calming fears.
Two pointers hop from right to left,
No character wasted, none bereft.
The strings compare — both match, hooray!
This rabbit coded well today. 🥕

🚥 Pre-merge checks | ✅ 4 | ❌ 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 (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately and concisely describes the main addition: a new backspace string comparison algorithm using the two-pointers technique.
Description check	✅ Passed	The pull request description is complete and comprehensive, covering the change description and addressing all checklist items with appropriate selections and confirmations.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/backspace-string-compare

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