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Quick Summary
Find all distinct communities that share at least one direct follower with a specified target community.
Leverage the bidirectional mapping between communities and followers to traverse exactly one relationship hop.
Gracefully handle invalid indices or empty follower lists by returning an empty result.
What This Tests
Single-hop graph traversal and neighbor discovery
Hash-based deduplication and set operations
Boundary validation and array/list indexing
Relationship mapping and data structure utilization
Common Patterns
Graph Traversal
Hashing
Sorting
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