[kobukuro]

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[Copilot]

Pull Request Overview

This PR implements a solution for the LeetCode "Destination City" problem (#1436) by replacing an inefficient HashMap-based approach with an optimized HashSet solution and adding comprehensive unit tests.

• Replaced complex HashMap implementation with a simpler and more efficient HashSet approach
• Added comprehensive unit tests covering multiple test cases including edge cases
• Moved the solution file to the proper package structure under src/main/java/com/leetcode/easy/

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/main/java/com/leetcode/easy/DestinationCity1436.java	New optimized solution using HashSet with O(n) time/space complexity and detailed comments
src/test/java/com/leetcode/easy/DestinationCity1436Test.java	Comprehensive unit tests covering various scenarios including single path edge case
src/DestinationCity1436.java	Removed old inefficient HashMap-based implementation from root directory

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