This is Floyd's cycle detection algorithm, named after Robert Floyd. It's a classic computer science technique that appears beyond just linked lists. Amazon, Google, and Microsoft all ask this. The space constraint is what makes it interesting. Anyone can solve it with a hash set. The fast/slow pointer technique shows you know classic algorithms. The follow-up "find where the cycle starts" is common. If they ask it, explain the math: when slow and fast meet, reset one to head and move both at the same speed. They'll meet at the cycle start.
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$ curl repovive.com/roadmaps/maang-interview-prep/linked-lists/linked-list-cycle-why-this-problem
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