Implement command-driven in-memory key-value database

Q: Implement command-driven in-memory key-value database

This is a Coding & Algorithms interview question from Lyft for Software Engineer roles. View the full question and solution on PracHub.

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Question

Implement a command-driven in-memory key–value database. Supported commands (one per line):

SET key value
GET key → print value or NULL
DELETE key
COUNT value → print how many keys currently map to this value
BEGIN → start a transaction
ROLLBACK → undo changes in the most recent open transaction; if none, print NO TRANSACTION
COMMIT → make all open transaction changes permanent. Constraints: up to 100,000 commands; keys and values are ASCII strings; average O(
per operation; memory must remain within reasonable bounds. Design data structures to support COUNT efficiently and implement transactional semantics using an undo log or equivalent. Specify I/O format precisely and discuss edge cases (nested transactions, deleting non-existent keys, multiple keys sharing a value).

Implement command-driven in-memory key-value database

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