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My first instinct was to use a hash of the URL as the key, which works until you realize collisions are a real problem and you need to handle them.
Clarify requirements (e.g., fixed prefix, expected O(1) time, in-memory) and then propose a design using a hash map for bidirectional mapping and a counter or random ID generator for unique short codes. Discuss trade-offs such as collision handling, scalability, and potential improvements like base62 encoding.
Pro tip: Mention that using a simple incrementing counter with base62 encoding guarantees uniqueness and O(1) operations, but be prepared to discuss the trade-off of predictable short URLs and potential security concerns.
Ask about expected scale, prefix format, character set for short codes, and whether persistence is needed. Confirm that both encode and decode must be O(1) on average.
Propose using two hash maps: one from long URL to short code and one from short code to long URL. Use a counter or random generator to create unique short codes.
For encode, check if long URL already exists; if not, generate a new short code, store mappings, and return the short URL. For decode, look up the short code in the map and return the long URL.
Explain that both operations are O(1) average time due to hash map lookups. Discuss trade-offs: counter approach is simple but predictable; random approach may need collision handling.
Mention how to handle collisions, scale to distributed systems, add expiration, or use a more compact encoding like base62.
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