Linked List Cycle

Given the head of a linked list, determine if the linked list contains a cycle. A cycle exists if some node in the list can be reached again by continuously following the next pointer. Return true if there is a cycle, or false otherwise.

The classic approach is Floyd's cycle-detection (two-pointer / "tortoise and hare") which uses O(1) extra space.

Example

Input: head = [3,2,0,-4] (cycle: tail connects back to index 1) → Output: true Input: head = [1,2] (cycle: tail connects back to index 0) → Output: true Input: head = [1] (no cycle) → Output: false

Constraints

• The number of nodes is in the range [0, 10⁴].
• -10⁵ ≤ Node.val ≤ 10⁵

v1 Limitation

The v1 test schema cannot express cyclic input; all test cases use acyclic lists where the expected answer is always false. v1.1 may extend the schema with a cycleIndex per test case to support cycle inputs.