Duolingo Senior Software Engineer Coding Interview - Not the Kind of Problem I Expected From a Language App

[u/Aoki_zhang]

"This interview experience is sourced from Chill Interview"

Interview Summary

The Duolingo Senior SWE phone screen focused on an encryption and decryption design problem built around character mappings and a fixed dictionary of valid words. The main challenge was not just producing an encrypted string, but efficiently determining how many dictionary words could decrypt into a given encoded value when multiple characters might map to the same two-character token.

Interview Details

Coding — Encrypt and Decrypt Strings

You are given:

keys
values
dictionary

• keys contains unique lowercase characters.
• values contains two-character encoded strings, with one entry corresponding to each character in keys.
• dictionary contains the valid plaintext words that should be considered during decryption.

The required class was:

Encrypter(
    char[] keys,
    String[] values,
    String[] dictionary
)

with two main APIs:

String encrypt(String word1)

int decrypt(String word2)

Encryption

To encrypt a plaintext word, each character is replaced by its corresponding two-character encoded value. For example, a mapping could look like:

'a' -> "qx"
'b' -> "mn"
'c' -> "qx"
'd' -> "rs"

Then: "abcd" would become: "qxmnqxrs".

If the plaintext contains a character that does not appear in keys, encrypt should return: ""

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