Duolingo coding interview
questions, leaked.
4 problems reported across recent Duolingo interviews. Top patterns: array, hash table, string. The list below is what most reported candidates actually saw, plus the honest play if you can't grind all of it.
Duolingo's coding interview is compact but brutal. Four problems total, one at hard difficulty. You'll see arrays in almost every single problem, hash tables in three of them, and the hard problem chains multiple patterns together: encrypt/decrypt with tries and design. This isn't a breadth test. It's depth. If you bomb the encrypt problem, you're sunk. StealthCoder sits invisible during your assessment and pulls a working solution the moment you're stuck, so you don't spiral on the hard one.
Top problems at Duolingo
| # | Problem | Diff | Frequency | Pass % | Patterns |
|---|---|---|---|---|---|
| 01 | Encrypt and Decrypt Strings | HARD | 100.0 | 37% | Array · Hash Table · String |
| 02 | Minimum Number of People to Teach | MEDIUM | 82.6 | 44% | Array · Hash Table · Greedy |
| 03 | K Radius Subarray Averages | MEDIUM | 80.9 | 46% | Array · Sliding Window |
| 04 | Task Scheduler II | MEDIUM | 52.1 | 54% | Array · Hash Table · Simulation |
Frequencies derived from public community-tagged interview reports. Click a row to view on LeetCode.
You have a week, maybe less. You can't out-grind the list above. StealthCoder runs invisibly during the actual Duolingo OA. The proctor cannot see it. Screen share cannot detect it. Built by an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share.
Get StealthCoder- array4 · 100%
- hash table3 · 75%
- string1 · 25%
- design1 · 25%
- trie1 · 25%
- greedy1 · 25%
- sliding window1 · 25%
- simulation1 · 25%
Arrays dominate here. You need to move fast on array fundamentals because three of your four problems depend on it. Hash tables are your second anchor, appearing alongside arrays in most medium problems. The hard problem, Encrypt and Decrypt Strings, mixes arrays, hash tables, tries, and design all at once. Study that one first. Sliding window, greedy, and simulation are supporting patterns, not primary threats. You'll likely have one or two medium problems that test whether you can handle basic optimization quickly. The gap between medium and hard is wide. StealthCoder is your safety net if the design logic or trie traversal stalls you mid-OA.
Companies with similar patterns
If you prepped for Duolingo, these companies recycle ~60% of the same topics.
You've seen the list.
Now make sure you pass Duolingo.
Memorizing every problem above in a week is a fantasy. StealthCoder is the hedge: an AI overlay that's invisible during screen share. It reads the problem on screen and surfaces a working solution in under 2 seconds. Built by an engineer at a top-10 tech company who can solve these problems cold but didn't want to trust himself in a 90-minute screen share. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Duolingo interview FAQ
How many array problems should I solve before my Duolingo OA?+
At least 8 to 12. Arrays appear in all four of Duolingo's problems, so you need speed and confidence here. Focus on subarray logic, prefix sums, and two-pointer patterns first. K Radius Subarray Averages and Minimum Number of People to Teach both demand solid array handling.
Is hash table knowledge required for this interview?+
Yes. Three of four problems use hash tables. You don't need to be a hash table expert, but you must be fluent in counting, grouping, and tracking frequency with maps or dictionaries. That's tested in both medium and hard problems.
Should I study tries before my OA?+
Only if you have time after mastering arrays and hash tables. The hard problem, Encrypt and Decrypt Strings, uses a trie, but if you're weak on tries, the other three medium problems won't punish you. Trie is a single point of failure, so know it well or accept the risk.
What's the hardest problem I'll face?+
Encrypt and Decrypt Strings. It combines arrays, hash tables, tries, and design logic. It's the only hard problem in the set. Spend time understanding the trie construction and traversal logic before your OA. This is where most candidates lose points.
How much time should I spend on sliding window and greedy?+
Limited. Sliding window shows up once in K Radius Subarray Averages. Greedy appears once in Minimum Number of People to Teach. Drill these after you're solid on arrays and hash tables. They're not the focus here.