Given the head of a singly linked list, return true if the list contains a cycle — some node's next pointer leads back to an earlier node instead of eventually reaching null. Since a cyclic list can never be fully printed or walked to completion, this test's inputs are described as (values, pos): build a normal list from the values, then point the last node's next at the node located at index pos (0-indexed). pos = -1 means no cycle at all. The optimal solution — Floyd's Cycle DetectionFloyd's Cycle DetectionAlso called the "tortoise and hare" algorithm. Two pointers move through the list at different speeds (1 step and 2 steps). If a cycle exists, both get trapped inside it and are guaranteed to eventually land on the same node — since the faster one gains exactly one node on the slower one every step. — detects this in O(1) extra space, without ever storing a single visited node.