Skip to main content

19. Linked List Cycle

easyAsked at Zoom

Detect whether a linked list contains a cycle.

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

Problem

Given head, the head of a linked list, return true if there is a cycle. There is a cycle if some node can be reached again by continuously following next pointers.

Constraints

  • 0 <= nodes <= 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], pos=-1
Output
false

Approaches

1. Visited set

Track visited nodes.

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

Tradeoff:

2. Floyd's tortoise and hare

Two pointers, slow and fast; they meet inside a cycle.

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:

Zoom-specific tips

Zoom uses cycle-detection mental models for their meeting-state graph (loops in reconnection logic); explain Floyd's invariants succinctly.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Linked List Cycle and other Zoom interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →