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13. Linked List Cycle

easyAsked at MercadoLibre

Detect whether a singly linked list contains a cycle.

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

Problem

Given the head of a linked list, return true if the list contains a cycle, otherwise false. A cycle exists when some node can be reached again by continuously following next pointers.

Constraints

  • 0 <= node count <= 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

Walk the list, marking each node in a set; return true on revisit.

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 & hare

Two pointers, one moving one step and one moving two. If they meet, there's 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:

MercadoLibre-specific tips

MercadoLibre logistics teams use cycle detection on routing graphs — courier handoff sequences must not loop, and they want to see the constant-space pointer pattern, not a visited-set crutch.

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