Detect cycle in linked list using Floyd algorithm
Reported in Datadog interview loops. Classic linked list pointer question in entry and mid-level interviews.
Interview scenario
Context for Datadog candidates:
Given the head of a linked list, determine whether it contains a cycle.
Model answer
Try answering aloud first
Cover trade-offs, structure, and a concrete example before revealing the baseline response.
How to frame this at Datadog: Connect your answer to measurable impact, clarity of thought, and trade-offs the team cares about. Below is a strong baseline response you can adapt with your own project examples.
Use two pointers: slow moves one step, fast moves two steps. If there is a cycle, fast eventually laps slow and both pointers meet; if fast reaches null, there is no cycle.
This approach avoids additional hash memory and runs in O(n) time and O(1) space. Interviewers also like a follow-up: after meeting, reset one pointer to head and move both one step to find the cycle start.
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