Linked List Cycle

The problem. Given head, determine whether the linked list has a cycle: some node whose next arrow points back to an earlier node, so walking the list would go round forever. Return true if there is a cycle, otherwise false.

The list 3 -> 2 -> 0 -> -4 where -4 points back to 2, forming a loop. Following the arrows never reaches null.

Example

Input:  head = [3, 2, 0, -4], tail connects to index 1
Output: true

Input:  head = [1], no cycle
Output: false

You can't just walk until null — with a cycle, null never comes. You need a way to notice that you've been somewhere before.

Free account

Keep reading. It's free.

Sign up to read the rest of this lesson: 5 more sections, 2 drawings, a dry-run simulator and code in JavaScript, Python, Java and C++.

Still to come

1. Brute force: remember every node, search the list of them
2. Optimal: a hash set of visited nodes
3. Best solution: Floyd's tortoise and hare
Comparing the three approaches
Edge cases to test

Prev Next

Mark completed

Was this helpful?

StackLast-in, first-out — the structure behind call stacks, undo history, and expression parsing.6 lessons · 13 problems
QueueFirst-in, first-out — the structure behind task scheduling and breadth-first traversal.4 lessons · 5 problems
ArraysContiguous, indexable memory — the foundation almost every other data structure builds on.12 lessons · 16 problems

Linked List Cycle

Example

Keep reading. It's free.

Keep exploring