How often are these problems asked?
Frequency scores are based on crowdsourced interview reports. A higher score means the problem has been reported more often in recent Lucid interviews.
Very Likely
75-100%
Likely
50-74%
Sometimes
25-49%
Rare
0-24%
Problem database last updated: June 20, 2025
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Lucid Coding Interview Questions
8 problems · 2 Easy, 5 Medium, 1 Hard · Ranked #174 of 458
Difficulty breakdown
2 Easy
25% · avg 23%
5 Medium
63% · avg 59%
1 Hard
13% · avg 18%
Top topics
array
37.5%
string
37.5%
hash-table
25%
stack
25%2.9x
tree
12.5%2.2x
graph
12.5%4.2x
Interview profile
Based on 8 reported problems, Lucid interviews are in line with industry averages - 13% Hard vs 18% overall. The majority (63%) of questions are Medium difficulty, which is typical for companies that want to see solid fundamentals without excessive trick questions.
Compared to the industry average, Lucid puts unusual emphasis on stack (25% of problems, 2.9x the industry average). If you're short on time, these are the categories to double down on.
The most common topics are array (37.5%), string (37.5%), hash-table (25%), stack (25%). Problems below are sorted by frequency, the ones at the top are asked most often.
All 8 problems
| Problem | Difficulty | Frequency | Topics | |
|---|---|---|---|---|
Collect Coins in a Tree There exists an undirected and unrooted tree with n nodes indexed from 0 to n - 1. You are given an integer n and a 2D integer array edges of length n - 1, wher... | Hard | Very Likely | arraytreegraph | Solve |
Maximum Number of Balls in a Box You are working in a ball factory where you have n balls numbered from lowLimit up to highLimit inclusive (i.e., n == highLimit - lowLimit + 1), and an infinite... | Easy | Very Likely | hash-tablemathcounting | Solve |
Min Stack Design a stack that supports push, pop, top, and retrieving the minimum element in constant time. | Medium | Likely | stackdesign | Solve |
Valid Parentheses Given a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid. | Easy | Likely | stringstack | Solve |
Generate Parentheses Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses. | Medium | Likely | stringdynamic-programmingbacktracking | Solve |
Number of Islands Given an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands. | Medium | Likely | arraydepth-first-searchbreadth-first-search | Solve |
Longest Substring Without Repeating Characters Given a string s, find the length of the longest substring without duplicate characters. | Medium | Likely | hash-tablestringsliding-window | Solve |
Gas Station There are n gas stations along a circular route, where the amount of gas at the ith station is gas[i]. | Medium | Likely | arraygreedy | Solve |
Collect Coins in a Tree
SolveThere exists an undirected and unrooted tree with n nodes indexed from 0 to n - 1. You are given an integer n and a 2D integer array edges of length n - 1, wher...
HardVery Likely
arraytreegraph
Maximum Number of Balls in a Box
SolveYou are working in a ball factory where you have n balls numbered from lowLimit up to highLimit inclusive (i.e., n == highLimit - lowLimit + 1), and an infinite...
EasyVery Likely
hash-tablemathcounting
Min Stack
SolveDesign a stack that supports push, pop, top, and retrieving the minimum element in constant time.
MediumLikely
stackdesign
Valid Parentheses
SolveGiven a string s containing just the characters '(', ')', '{', '}', '[' and ']', determine if the input string is valid.
EasyLikely
stringstack
Generate Parentheses
SolveGiven n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.
MediumLikely
stringdynamic-programmingbacktracking
Number of Islands
SolveGiven an m x n 2D binary grid grid which represents a map of '1's (land) and '0's (water), return the number of islands.
MediumLikely
arraydepth-first-searchbreadth-first-search
Longest Substring Without Repeating Characters
SolveGiven a string s, find the length of the longest substring without duplicate characters.
MediumLikely
hash-tablestringsliding-window
Gas Station
SolveThere are n gas stations along a circular route, where the amount of gas at the ith station is gas[i].
MediumLikely
arraygreedy
Preparing for your Lucid coding interview
Lucid interviews focus heavily on array, string, hash-table problems. If you're short on time, these are the categories to prioritize. The problems on this page are sorted by frequency, so start from the top and work your way down.
Beyond solving problems, practice explaining your approach. Lucid interviewers care about your thought process - how you break down a problem, consider edge cases, and evaluate tradeoffs between solutions. A clean O(n) solution you can explain clearly beats an O(log n) solution you can't articulate.
Looking for more companies? Browse all 458 companies in our directory, or sharpen your fundamentals with our free data structure visualizers and AI-powered DSA tutor.
Frequently Asked Questions
What coding problems does Lucid ask in interviews?add
Lucid has been reported to ask 8 distinct coding problems. The most common topics are array, string, hash-table. 2 are Easy difficulty, 5 are Medium, and 1 are Hard. Problems are sorted by frequency - the ones at the top are asked most often.
How hard are Lucid coding interviews?add
Based on 8 reported problems, Lucid interviews are in line with industry averages - 13% Hard vs 18% overall. 63% of questions are Medium difficulty. Focus on the high-frequency Medium problems first, then work through the Hard ones.
How should I prepare for a Lucid coding interview?add
Start with the highest-frequency problems listed on this page. Focus on the core topics: array, string, hash-table. Practice solving them under time pressure and explaining your approach out loud. Mock interviews with AI can simulate the real experience.
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