Find cycle in linked list
Last updated: July 1, 2025
Quick Overview
Given a singly linked list, determine if it contains a cycle in it. A cycle is defined as a situation where a node's next pointer points back to a previous node in the list. Your function should return true if a cycle exists, and false otherwise, with a time complexity of O(n) and a space complexity of O(1).
DoorDash
July 1, 202575
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1,859 solved
Given a singly linked list, determine if it contains a cycle in it. A cycle is defined as a situation where a node's next pointer points back to a previous node in the list. Your function should return true if a cycle exists, and false otherwise, with a time complexity of O(n) and a space complexity of O(1).
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 your solution change if the input was sorted?
- Can you solve this in a single pass?
- How would you modify your solution to handle streaming input?
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Practice DSA ProblemsSample Answer
Problem Analysis
To determine if a linked list contains a cycle, we can utilize the Floyd's Tortoise and Hare algorithm, which employs the two pointers technique. This method is suitable here because it allows...
Approach
- Initialize Two Pointers: Start with two pointers,
slowandfast, both at the head of the linked list. - Traverse the List: Move
slowby one step andfastby two steps in each itera...
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