Applied intuition·Software Engineer·Technical Phone Screen·Senior

Senior

Jun 2026

Summary

Applied Intuition coding round for a software engineer role. One meaty design-and-implement question that took up the whole session, covering transactions, nested contexts, and complexity tradeoffs. More system-flavored than I expected for a coding interview.

Questions Asked (1)

Design and implement an in-memory key-value store that supports transactions and arbitrarily nested transactional blocks. The system should handle SET, GET, BEGIN, ROLLBACK, and COMMIT commands, where ROLLBACK discards the current transaction's changes and COMMIT merges all open transactions into the base state. Also discuss the time and space complexity of each command, compare a stack-based versus recursive approach for managing nested transactions, and walk through example interactions including edge cases like GET on a missing key or ROLLBACK with no open transaction.

Algorithms & Data StructuresSystem DesignTechnical Trade-offs

Author's notes

This one took me a second to parse because it sounds like a database question but it's really a stack management problem.

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AI HintsAI Generated

Suggested Approach

Start by clarifying requirements and defining the data model, then explain the stack-based approach for nested transactions, implement each command with complexity analysis, and walk through examples including edge cases. Compare stack-based vs recursive approaches, highlighting trade-offs in simplicity, performance, and memory.

Pro tip: Emphasize that COMMIT merges only the current transaction's changes into its parent, not directly into the base state, to maintain isolation and correctness. Also, mention that GET should check the transaction stack from top to bottom to respect uncommitted changes.

1. Clarify Requirements and Data Model

Confirm that transactions can be nested arbitrarily, and that COMMIT merges changes into the immediate parent transaction. Define the base store as a hash map and each transaction as a stack of change sets.

2. Design the Transaction Stack

Use a stack (list) to track active transactions. Each transaction stores a map of key-value changes (or deletions) made within it. BEGIN pushes a new empty transaction onto the stack.

3. Implement Commands with Complexity Analysis

For SET, update the top transaction's map (O(1) time, O(1) space per change). For GET, search from top of stack down to base (O(depth) time). ROLLBACK pops the top transaction (O(1) time, O(k) space freed). COMMIT merges top transaction into the one below (O(k) time, O(k) space).

4. Compare Stack-Based vs Recursive Approaches

Stack-based is iterative, uses explicit memory, and is easy to reason about. Recursive uses call stack, may risk stack overflow for deep nesting, and is less flexible for merging. Stack-based is generally preferred for clarity and control.

5. Walk Through Examples and Edge Cases

Demonstrate a sequence: SET a 1, BEGIN, SET a 2, GET a (returns 2), ROLLBACK, GET a (returns 1). Show GET on missing key returns null. Show ROLLBACK with no open transaction is a no-op or error. Show COMMIT with no open transaction is a no-op or error.

Key Points to Mention

Time complexity: SET O(1), GET O(depth), BEGIN O(1), ROLLBACK O(1), COMMIT O(k) where k is number of changes in the transaction.
Space complexity: O(total number of uncommitted changes across all transactions).
Stack-based approach uses an explicit stack of transactions, each with its own change map.
Recursive approach uses function call stack, but merging on commit is more complex and may cause stack overflow.
Edge cases: GET on missing key returns null/None; ROLLBACK or COMMIT with no open transaction should be handled gracefully (no-op or error).
COMMIT merges changes into the parent transaction, not directly into the base store, to preserve isolation.

AI-generated suggestions, not part of the candidate's original notes. May be inaccurate — verify before relying on them.