Duolingo·Software Engineer·Technical Phone Screen·Intermediate

Intermediate

May 2026

Summary

Duolingo software engineer interview with a coding round centered on a string encryption/decryption design problem. The question looked straightforward at first but the decrypt side has a real gotcha that caught me off guard.

Questions Asked (1)

Design and implement an Encrypter/Decrypter class. You're given a 1-to-1 mapping from single characters to two-character strings. encrypt() replaces each character with its mapped value. decrypt() returns how many words from a given dictionary, when encrypted, produce the target string.

Algorithms & Data StructuresTechnical Trade-offsSystem Design

Author's notes

The encrypt direction was trivial, just a hash map lookup and string concat.

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AI HintsAI Generated

Suggested Approach

Start by clarifying the mapping and the dictionary-based decryption requirements, then design a class with encrypt and decrypt methods. For decrypt, use backtracking to count how many dictionary words, when encrypted, match the target string, and discuss trade-offs like precomputing encrypted dictionary or using memoization.

Pro tip: Mention that since the mapping is 1-to-1, the encrypted string length is always twice the original, and you can prune the backtracking by checking if the remaining target length can be formed by the remaining characters.

1. Clarify requirements and constraints

Ask about the size of the dictionary, the length of words, and whether the mapping is fixed or provided. Confirm that decrypt counts words whose encryption equals the target string.

2. Design the Encrypter/Decrypter class

Define a class with a constructor that takes the character-to-two-character mapping. Implement encrypt by iterating through the input string and replacing each character with its mapped value.

3. Implement decrypt using backtracking

Use a recursive function that tries to match the target string by consuming two characters at a time and checking if they correspond to a character in the mapping. For each valid character, recursively process the remainder and count matches from the dictionary.

4. Optimize with precomputation or memoization

Precompute the encrypted form of each dictionary word and store them in a set for O(1) lookup. Alternatively, use memoization on the target string to avoid redundant recursive calls.

5. Analyze complexity and trade-offs

Discuss time and space complexity. Backtracking without optimization is exponential in the worst case, but precomputation and pruning can improve performance. Mention that the 1-to-1 mapping ensures unique decryption of characters.

Key Points to Mention

The mapping is 1-to-1, so each character maps to a unique two-character string, and decryption is unambiguous at the character level.
Encryption is straightforward: iterate through the string and concatenate mapped values.
Decryption requires counting dictionary words whose encryption equals the target, which can be solved by backtracking over the target string.
Precompute encrypted dictionary words and store in a hash set for efficient lookup.
Use memoization to cache results for substrings of the target to avoid redundant computations.
Prune the search space by checking if the remaining target length is even and can be formed by remaining characters.

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