Find cycle in linked list
Last updated: December 19, 2025
Quick Overview
Given a singly linked list, determine if it contains a cycle in it. If a cycle exists, return the node where the cycle begins; otherwise, return null. You should aim for an efficient solution with O(n) time complexity and O(1) space complexity.
Doordash
December 19, 20259
11
3,466 solved
Given a singly linked list, determine if it contains a cycle in it. If a cycle exists, return the node where the cycle begins; otherwise, return null. You should aim for an efficient solution with O(n) time complexity and O(1) space complexity.
Coding interviews at Doordash 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 iteratively instead of recursively (or vice versa)?
- How would you parallelize this solution?
- Can you optimize the space complexity of your solution?
- Can you solve this in a single pass?
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Practice DSA ProblemsSample Answer
Problem Analysis
This problem can be effectively solved using the Floyd's Tortoise and Hare algorithm, which utilizes the two pointers technique. The reason this applies here is that we need to detect a cycle ...
Approach
- Initialize Two Pointers: Start two pointers,
slowandfast. Setslowto the head of the linked list andfastto the head as well. - Traverse the List: Move
slowone step at a ti...
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