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9. Linked List Cycle

easyAsked at Squarespace

Detect whether a singly linked list contains a cycle; Squarespace uses it to gauge whether you know Floyd's two-pointer technique versus building a visited set.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the head of a linked list, determine if the list has a cycle in it. A cycle exists if some node can be reached again by continuously following the next pointer.

Constraints

  • 0 <= number of nodes <= 10^4
  • -10^5 <= Node.val <= 10^5

Examples

Example 1

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

Example 2

Input
head=[1,2], pos=-1
Output
false

Approaches

1. Visited set

Walk the list and bail out as soon as a node repeats.

Time
O(n)
Space
O(n)
const seen = new Set();
let n = head;
while (n) { if (seen.has(n)) return true; seen.add(n); n = n.next; }
return false;

Tradeoff:

2. Floyd's tortoise and hare

Two pointers, one stepping by one and one by two, must collide inside a cycle. Constant extra memory.

Time
O(n)
Space
O(1)
function hasCycle(head) {
  let slow = head, fast = head;
  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;
    if (slow === fast) return true;
  }
  return false;
}

Tradeoff:

Squarespace-specific tips

Squarespace reviewers like the link to their template-block dependency graph, where cycle detection guards against infinite include loops at publish time.

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Output

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