Lucid coding interview
questions, leaked.
8 problems reported across recent Lucid interviews. Top patterns: string, array, stack. The list below is what most reported candidates actually saw, plus the honest play if you can't grind all of it.
Lucid's interview is heavy on string and array problems, with stack patterns showing up hard and fast. You're looking at 8 questions total, split 2 easy, 5 medium, 1 hard. That hard one, Collect Coins in a Tree, is a graph traversal nightmare. The mediums are where you'll actually spend your time: string manipulation with hash tables, stack design, greedy choices on arrays. If you blank on a sliding-window or stack-inversion problem during the live assessment, StealthCoder surfaces a working solution in seconds, invisible to the proctor. The payoff is real: you move past panic and nail the next one.
Top problems at Lucid
| # | Problem | Diff | Frequency | Pass % | Patterns |
|---|---|---|---|---|---|
| 01 | Maximum Number of Balls in a Box | EASY | 100.0 | 74% | Hash Table · Math · Counting |
| 02 | Collect Coins in a Tree | HARD | 100.0 | 36% | Array · Tree · Graph |
| 03 | Valid Parentheses | EASY | 73.8 | 42% | String · Stack |
| 04 | Min Stack | MEDIUM | 73.8 | 56% | Stack · Design |
| 05 | Generate Parentheses | MEDIUM | 64.9 | 77% | String · Dynamic Programming · Backtracking |
| 06 | Gas Station | MEDIUM | 64.9 | 46% | Array · Greedy |
| 07 | Longest Substring Without Repeating Characters | MEDIUM | 64.9 | 37% | Hash Table · String · Sliding Window |
| 08 | Number of Islands | MEDIUM | 64.9 | 62% | Array · Depth-First Search · Breadth-First Search |
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 Lucid OA. The proctor cannot see it. Screen share cannot detect it. Built by an Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code.
Get StealthCoder- string3 · 38%
- array3 · 38%
- stack2 · 25%
- hash table2 · 25%
- design1 · 13%
- dynamic programming1 · 13%
- backtracking1 · 13%
- greedy1 · 13%
- math1 · 13%
- counting1 · 13%
String and array dominate this dataset, each appearing 3 times. Stack hits twice and is split between validation (Valid Parentheses, easy) and design (Min Stack, medium). Hash-table problems mix string work with math. The medium tier is where Lucid separates confident from reckless: Generate Parentheses requires backtracking; Gas Station demands greedy intuition; Longest Substring Without Repeating Characters needs a sliding-window hash map. The hard question, Collect Coins in a Tree, combines array, tree, and graph traversal with a topological sort angle. Your prep order matters: nail Valid Parentheses and the string-hash-table patterns first, then move to design and greedy. StealthCoder is your hedge for the medium-to-hard jump if you haven't drilled topological sort or backtracking edge cases before the OA.
Companies with similar patterns
If you prepped for Lucid, these companies recycle ~60% of the same topics.
You've seen the list.
Now make sure you pass Lucid.
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 Amazon engineer who realized the OA tests how well you memorized 200 problems, not how well you code. Works on HackerRank, CodeSignal, CoderPad, and Karat.
Lucid interview FAQ
How many string problems should I drill before a Lucid interview?+
String appears in 3 of 8 reported problems. Drill Valid Parentheses, Generate Parentheses, and Longest Substring Without Repeating Characters. That covers easy, medium backtracking, and medium sliding-window. Those three patterns alone unlock the string tier.
Is the hard problem actually part of the interview loop?+
Collect Coins in a Tree is reported as a hard question and shows up in the data. It combines tree, graph, and topological sort. You may not see it, but if you do, it's a signal you're in a later round or higher-level loop. Prep it last, after mediums.
What should I prioritize, stack or array problems?+
Array appears 3 times, stack 2. But both are critical. Stack problems test design thinking and inversion logic; array problems test greedy and traversal. Start with Valid Parentheses and Min Stack to build confidence, then move to Gas Station and Number of Islands.
How important is hash-table knowledge for Lucid?+
Hash tables appear in 2 reported problems and support string work in several others. Longest Substring Without Repeating Characters and Maximum Number of Balls in a Box both require hash-map speed. It's foundational, not optional.
Should I learn topological sort before the interview?+
One hard problem, Collect Coins in a Tree, uses it. If you have a week, learn the basics. If you have days, skip it and trust that a medium will land instead. That's the bet most candidates make.