Problem - Linked List Cycle

Given the head of a linked list, determine if the list has a cycle. A cycle exists if some node's next pointer points back to a previous node, creating a loop.

If you traverse naively, you'll loop forever. Example: $1 \to 2 \to 3 \to 4 \to 2$ (4 points back to 2) has a cycle. The naive approach uses a hash set to track visited nodes.

$O(n)$ space. Can you detect cycles in $O(1)$ space? Constraints: up to $10^4$ nodes.