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Duolingo·Software Engineer·Technical Phone Screen·Intermediate

Intermediate
Apr 2026

Summary

One round at Duolingo for a software engineer role, just a single coding question that was close to a known LeetCode problem but with a twist on the output.

Questions Asked (1)

Q1

Given an encrypted string and a key, decrypt it and return the full decoded string (not just a count).

Algorithms & Data Structures
Author's notes

Recognized the base problem pretty fast since it's close to something I'd seen before.

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AI HintsAI Generated

Suggested Approach

First, clarify the encryption scheme and constraints (e.g., character set, key usage, edge cases) with the interviewer. Then, outline a step-by-step decryption algorithm, focusing on efficiency and correctness, and walk through a small example to validate your logic before coding.

Pro tip: Demonstrate thoroughness by discussing how you would handle edge cases like empty strings, invalid keys, or non-alphanumeric characters, and mention potential optimizations or alternative approaches.

1. Clarify the problem

Ask questions to understand the encryption method, key format, expected input/output, and constraints (e.g., time/space complexity, character set).

2. Outline the algorithm

Describe the high-level steps to decrypt the string, such as iterating through characters, applying the key, and building the result.

3. Walk through an example

Choose a small encrypted string and key, and manually step through your algorithm to verify it produces the correct decoded string.

4. Code the solution

Write clean, modular code, handling edge cases and using appropriate data structures.

5. Test and optimize

Test with various inputs, including edge cases, and discuss potential optimizations or trade-offs.

Key Points to Mention

  • Understanding of common encryption/decryption techniques (e.g., Caesar cipher, XOR, Vigenère) and how keys are applied.
  • Handling of edge cases: empty string, key length, special characters, and invalid inputs.
  • Time and space complexity analysis of the proposed solution.
  • Use of appropriate data structures (e.g., StringBuilder for efficient string concatenation).
  • Modular and readable code with clear variable names and comments.
  • Testing strategy: unit tests, boundary cases, and validation against known examples.

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