Linked List Cycle - Floyd's Algorithm

Use two pointers: slow moves $1$ step, fast moves $2$ steps. If there's a cycle, they will eventually meet.

$1.$ Initialize both pointers at head.

$2.$ Move slow by $1$, fast by $2$.

$3.$ If fast reaches null, no cycle.

$4.$ If slow meets fast, there's a cycle.

Why it works: in a cycle, fast gains $1$ step per iteration. If the cycle has length $L$, they meet within $L$ iterations.