Upstart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Coding round at Upstart for a software engineer role, pretty much one meaty problem around string decryption and cipher logic. Not the hardest thing I've done but the edge cases caught me slipping.

Questions Asked (1)

Given an encrypted string and a character-mapping rule, implement a decryption function. Handle edge cases like unmapped characters, case sensitivity, ambiguous decodings, and input validation.

Algorithms & Data StructuresTechnical Trade-offs

Author's notes

I went straight to the basic substitution case and got that working fine.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the problem: confirm the mapping direction (encrypted to decrypted), the data types, and how to handle unmapped characters. Then outline a step-by-step algorithm that validates input, applies the mapping, and resolves ambiguities, discussing trade-offs for each decision.

Pro tip: Explicitly state your assumptions about ambiguous decodings and unmapped characters, and propose a configurable policy (e.g., throw error, skip, or use placeholder) to show you think about real-world robustness.

1. Clarify requirements and edge cases

Ask questions to confirm the mapping direction, expected behavior for unmapped characters, case sensitivity rules, and what constitutes ambiguous decoding. Define input validation criteria.

2. Design the algorithm

Choose an appropriate data structure (e.g., hash map) for the mapping. Outline steps: validate input, iterate through the encrypted string, apply mapping, and handle special cases.

3. Handle edge cases explicitly

Describe how you will handle unmapped characters (e.g., throw exception, skip, or replace), case sensitivity (e.g., normalize case or treat separately), and ambiguous decodings (e.g., detect and resolve via context or error).

4. Implement and test

Write clean code with clear variable names and comments. Test with normal cases, edge cases (empty string, all unmapped, mixed case), and ambiguous scenarios.

5. Discuss trade-offs and improvements

Talk about time/space complexity, alternative approaches (e.g., using arrays for ASCII), and how to extend the solution for different policies or larger inputs.

Key Points to Mention

Input validation: check for null, empty string, and valid mapping structure.
Mapping direction: ensure you decrypt (encrypted -> decrypted) and not the reverse.
Unmapped characters: decide on a policy (error, skip, placeholder) and justify it.
Case sensitivity: clarify if mapping is case-sensitive and handle accordingly (e.g., normalize or separate maps).
Ambiguous decodings: identify scenarios where multiple decryptions are possible and propose a resolution strategy.
Complexity analysis: time O(n) and space O(1) or O(k) for mapping, and discuss trade-offs.

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