How do you detect a cycle in a Linked List? (Floyd's Tortoise and Hare)

0-2 years experience

1. 1Given a singly linked list implementation, how would you check if it contains a cycle using only O(1) extra space?
2. 2If you run the two‑pointer algorithm on a list that has a self‑loop at the head, what will happen and why?
3. 3What would you return when you detect a cycle, and how could you extend the function to also return the start node of the cycle?

2-5 years experience

1. 1We have a production service that processes a stream of events stored in a linked list; recently we saw occasional infinite loops. Walk me through how you'd debug and confirm a cycle using Floyd's algorithm.
2. 2Suppose you need to detect cycles but also count the number of nodes in the loop. How would you extend the tortoise‑hare approach?
3. 3If the list can be modified, would you consider breaking the cycle after detection? What trade‑offs are involved?

5-8 years experience

1. 1Our distributed cache uses a custom linked structure for LRU eviction across shards. How would you ensure cycle detection scales and doesn't become a bottleneck?
2. 2When integrating a third‑party library that provides its own linked list, you notice memory leaks due to hidden cycles. How would you design a wrapper that safely detects and handles cycles?
3. 3Discuss the performance implications of using Floyd's algorithm versus a hash‑set for cycle detection in a high‑throughput system, and when you might choose one over the other.

8+ years experience

1. 1We are refactoring a legacy codebase where many modules use hand‑rolled linked lists that may contain cycles. How would you architect a migration strategy that introduces a unified cycle‑detection utility while minimizing risk across teams?
2. 2Consider a microservices architecture where messages form a logical linked list across services, and cycles can cause deadlocks. How would you design a system‑wide contract or monitoring approach to detect and prevent cycles?
3. 3What guidelines would you set for future data‑structure design to avoid hidden cycles, and how would you enforce them across multiple engineering teams?