How do you detect a cycle in a linked list?

The optimal approach is Floyd's Cycle Detection (tortoise and hare) algorithm using two pointers: slow moves one step at a time, fast moves two steps. If there's a cycle, they will eventually meet; if fast reaches null, there's no cycle. Time complexity is O(n), space is O(1). To find the cycle start, after detection, move one pointer to head and advance both by one until they meet again.