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18. Linked List Cycle

easyAsked at Adyen

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, determine if the linked list has a cycle in it.

Constraints

  • 0 <= list length <= 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. Hash set visited

Track visited nodes in a set.

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

Tradeoff:

2. Floyd tortoise-and-hare

Slow and fast pointers eventually 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:

Adyen-specific tips

Adyen reaches for Floyd's variant because their retry-routing graphs must detect loops without allocating per-node bookkeeping in hot payment paths.

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