Linked List Cycle

Problem

Given the head of a linked list, return whether the list contains a cycle — some node's next pointing back to an earlier node, so traversal never reaches null.

Examples

Constraints

0 ≤ length ≤ 10⁴ · follow-up: O(1) memory.

Attempt it first

With the module's tools, the O(n)-space answer should come instantly (which verb from the Hash Tables module?). The O(1)-space answer is Floyd's tortoise-and-hare — you have both runners already; the work here is convincing yourself the meeting is guaranteed, not lucky. Try to argue it before opening the hints.