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19. Linked List Cycle

easyAsked at Slack

Determine whether a singly linked list contains a cycle.

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

Problem

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

Constraints

  • Number of nodes in [0, 10^4]
  • -10^5 <= Node.val <= 10^5

Examples

Example 1

Input
head = [3,2,0,-4], pos = 1
Output
true

Example 2

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

Approaches

1. Visited set

Track nodes seen in a Set; a revisit means a cycle.

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

Move slow by 1 and fast by 2. If they meet, there is a cycle. Constant space.

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:

Slack-specific tips

Slack relevant context — pubsub channel-listener chains can cycle; interviewers grade whether you mention loop-detection in async dispatchers.

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