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Upstart·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
May 2026

Summary

Coding round at Upstart for a software engineer role. One problem, fairly focused, centered on string decoding with chained hash map lookups. Not a brutal interview but the constraint framing kept me second-guessing myself.

Questions Asked (1)

Q1

Given a ciphertext string and two HashMaps, decode the string by mapping each character through the first map and then mapping that result through the second map. Implement this in O(n) time.

Algorithms & Data StructuresTechnical Trade-offs
Author's notes

Two lookups per character, so I kept overthinking whether chaining them violated the O(n) requirement.

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AI HintsAI Generated

Suggested Approach

Clarify the problem constraints and confirm the mapping semantics (e.g., what happens if a character is missing in a map). Then outline a single-pass O(n) algorithm using StringBuilder, and discuss trade-offs like space complexity and error handling.

Pro tip: Mention that you would use a StringBuilder instead of string concatenation to avoid O(n^2) time, and proactively discuss how to handle missing mappings (e.g., throw an exception or leave unchanged) to show attention to edge cases.

1. Clarify requirements and edge cases

Ask about the input format, whether maps are guaranteed to contain all characters, and what to do if a mapping is missing (e.g., throw exception, skip, or use default).

2. Design the algorithm

Explain that you will iterate through each character of the ciphertext, apply the first map, then the second map, and append the result to a StringBuilder. This ensures O(n) time.

3. Analyze complexity

State that time complexity is O(n) because each character is processed once, and space complexity is O(n) for the output string (plus O(1) auxiliary space if using StringBuilder).

4. Discuss trade-offs and alternatives

Mention that using a char array or pre-allocated StringBuilder can optimize memory, and that if maps are large, the O(1) lookup is still efficient. Also discuss error handling strategies.

5. Implement and test

Write clean code with meaningful variable names, handle null inputs, and walk through a simple example to verify correctness.

Key Points to Mention

  • Use StringBuilder for efficient string concatenation to maintain O(n) time.
  • Handle missing mappings explicitly (e.g., throw IllegalArgumentException or leave character unchanged) and discuss the choice.
  • Time complexity: O(n) where n is the length of the ciphertext; space complexity: O(n) for the output.
  • Consider using a char array if the output length is known and performance is critical.
  • Test with edge cases: empty string, null maps, characters not in maps.
  • Mention that the order of mapping matters: first map then second map.

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