Detect a cycle in a linked list, and find where the cycle starts.
Floyd's tortoise and hare finds a cycle in O(1) space, but what sets strong candidates apart is the second phase: a brief distance argument that identifies exactly where the loop starts. Here is the answer and the math.
Updated Sep 2026 · Grounded in real GenAI, LLM, and AI/ML engineering interview loops and written to a senior-engineer editorial bar.
Floyd's tortoise and hare finds a cycle in O(1) space, but what sets strong candidates apart is the second phase: a brief distance argument that identifies exactly where the loop starts. Here is the answer and the math.
Lead with where the obvious approach breaks, because that is the judgment they are screening for — most candidates jump straight to the happy path and lose the room.
Then walk the failure back through the pipeline in order, naming the one metric the customer's exec sponsor actually cares about before you propose the fix.
