[YTLim-1206]

Summary

• use deque to maintain BFS queue
• return the length of the shortest transformation sequence, or 0 if no valid transformation exists
• include the example to demonstrate the solution

Related issue

Closes #238

Checklist

• [✅] The solution includes the required problem, source, and note header comment.
• [✅] The file is in the correct solutions/<language>/ folder and follows the practice<n> naming convention.
• [✅] I ran the solution and checked that it works.
• [✅] I have kept this pull request focused on one change.

[Huerte]

@YTLim-1206 Hi there! Thank you for this contribution.