Add word ladder algorithm in backtracking

[Hardvan]

Describe your change:

Implemented the Word Ladder algorithm in the Backtracking folder.

The Word Ladder problem involves transforming a start word into a target word by changing one letter at a time, where each intermediate word must be a valid word from a given list. The challenge is to find a transformation sequence from the start word to the end word. In this backtracking approach, we attempt to explore all possible transformations, ensuring that each transformation is valid and leading us towards the end word.

Algorithm:

1. Start with the begin_word and recursively explore all valid transformations.
2. At each step, change one letter at a time to generate new words.
3. If the transformation results in the end_word, the process is complete, and the transformation sequence is returned.
4. If no transformation is possible, return an empty list.
5. Backtracking ensures that the algorithm explores all potential paths and retracts when a dead-end is reached.

• Add an algorithm?
• Fix a bug or typo in an existing algorithm?
• Add or change doctests? -- Note: Please avoid changing both code and tests in a single pull request.
• 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.
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Repository:

• Contributing guidelines
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Python:

• Formatted string literals (f-strings)
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• doctest
• unittest
• pytest

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[tianyizheng02]

Thanks for your contribution and for your changes!

[Hardvan]

Thanks for the helpful feedback and merging my PR!