Find cycle in graph
Last updated: June 6, 2026
Quick Overview
Given a directed or undirected graph represented as an adjacency list, determine if there is a cycle present in the graph. Your solution should return a boolean value: true if a cycle exists and false otherwise. Implement your algorithm using Depth-First Search (DFS) to ensure efficiency.
Lyft
June 6, 20262
16
2,258 solved
Given a directed or undirected graph represented as an adjacency list, determine if there is a cycle present in the graph. Your solution should return a boolean value: true if a cycle exists and false otherwise. Implement your algorithm using Depth-First Search (DFS) to ensure efficiency.
Coding interviews at Lyft focus on problem-solving approach as much as the final solution. The interviewer wants to see you break down the problem, consider edge cases, and optimize iteratively. Communication throughout the process is key.
What the Interviewer Expects
- Quickly identify the optimal approach and its theoretical basis
- Handle complex algorithm design with multiple interacting components
- Write concise, elegant code under time pressure
- Prove correctness of your approach and discuss alternative solutions
- Optimize beyond the obvious: discuss constant factor improvements
- Address follow-up variations and explain how the solution generalizes
Key Topics to Cover
How to Approach This
- Clarify input constraints and edge cases before writing code.
- Walk through your approach verbally and confirm with the interviewer before coding.
- Start with a brute force solution, then optimize. Mention time and space complexity.
- Test your solution with examples, including edge cases like empty input or duplicates.
- Consider common patterns: sliding window, two pointers, hash map, BFS/DFS, dynamic programming.
Possible Follow-up Questions
- Can you solve this in a single pass?
- What if the input doesn't fit in memory?
- How would your solution change if the input was sorted?
- How would you test this solution thoroughly?
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Problem Analysis
To determine if there is a cycle in a directed or undirected graph, we can utilize Depth-First Search (DFS). The key pattern here is that we can track the nodes visited during the DFS traversal, and f...
Approach
- Initialize Data Structures: Create a
visitedset to keep track of visited nodes and arecStackset to track nodes in the current path. - DFS Function: Implement a recursive DFS functi...
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