Duolingo Coding Interview Questions
25 Duolingo coding interview problems with full optimal solutions — 17 easy, 6 medium, 2 hard. Every problem ships with multiple approaches (brute-force first, then the optimal), complexity tables for each, company-specific tips on what an Duolingo interviewer values, and a FAQ section.
Showing 6 problems of 25
- #3mediumfoundational
3. Longest Substring Without Repeating Characters
Find the longest contiguous substring with all unique characters — the sliding-window pattern Duolingo applies when scanning a learner's typed sentence for the longest unique-token run to validate a free-form translation exercise.
- #49mediumfoundational
49. Group Anagrams
Cluster strings that are anagrams of each other into groups — the hash-keying technique Duolingo's vocabulary engine uses to bundle conjugation variants of the same root word into a single learning card set.
- #127mediumfoundational
127. Word Ladder
Find the shortest transformation sequence from one word to another by changing one letter at a time — the BFS word-graph search that mirrors how Duolingo's word-learning path finds the fewest morphological steps between a learner's current vocabulary and a target word.
- #208mediumfoundational
208. Implement Trie (Prefix Tree)
Build a trie that supports insert, search, and prefix-match — the core data structure powering Duolingo's autocomplete, spell-check hints, and word-bank prefix filtering that learners interact with on every vocabulary exercise.
- #322mediumfoundational
322. Coin Change
Find the minimum number of coins to reach an exact amount — the bottom-up DP pattern that directly models Duolingo's streak-recovery logic, where learners spend XP gems in minimum moves to restore a broken streak to its target length.
- #347mediumfoundational
347. Top K Frequent Elements
Return the k most frequent elements from an array — the heap or bucket-sort technique Duolingo's recommendation engine uses to surface the k words a learner encounters most so they can be promoted to the next spaced-repetition review slot.