Detect Loop in Linked List Visualization & Animation

Detects a cycle in a linked list using Floyd's fast-slow pointer algorithm; O(n) time, O(1) space.

## What is it? Detect whether a singly linked list contains a cycle (loop). Uses Floyd's Cycle Detection algorithm (fast and slow pointers) in O(1) space. ## How it works - Start `slow` and `fast` both at `head` - Move `slow` one step, `fast` two steps per iteration - If `fast` or `fast.next` becomes `null` → no cycle (list is finite) - If `slow == fast` at any point → a cycle exists (they met inside the cycle) **Why it works:** In a cycle, the fast pointer gains one step on the slow pointer each iteration; they must eventually meet. ## When to use - Detecting infinite loops in linked lists - As the first step of "Find Cycle Start" problems - Validating integrity of pointer structures ## Key Points - O(n) time — they meet in at most `n` steps - O(1) space — no visited set needed - Extension: to find where the cycle starts, see the "Find Cycle Start" algorithm

Category: algorithms

Difficulty: beginner

Time Complexity: O(n)

Space Complexity: O(1)

Detect Loop in Linked List

beginner

Detects a cycle in a linked list using Floyd's fast-slow pointer algorithm; O(n) time, O(1) space.

GFGLeetCode
Initializing
Iterations: 0
cycle138471204221563847349212156415674921530121567621563012slowfast
Both pointers start at the head — node 1. Slow moves one step per iteration; fast moves two. If there's a cycle, fast will eventually lap slow and they'll meet.