How would you check if a linked list has a cycle?

Use Floyd's cycle detection (tortoise and hare):

1. Start slow and fast pointers at the head.
2. Move slow one step and fast two steps each iteration.
3. If they meet, a cycle exists.
4. If fast reaches null, no cycle.

Time O(n), space O(1).

Alternative: store visited nodes in a Set — simpler but O(n) space.