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11. Linked List Cycle

easyAsked at Mercury

Detect whether a 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. A cycle exists when a node's next pointer points back to a previously visited node, forming a loop.

Constraints

  • Node count 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

Walk the list and store node references in a Set.

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

Tradeoff:

2. Floyd's tortoise and hare

Slow advances one step, fast two; if there's a cycle they collide. Constant space and linear time.

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:

Mercury-specific tips

Mercury reuses this to test cycle detection in transfer chains — circular money movements between subsidiaries trigger KYC flags, so cycle-detect logic must run before posting any wire.

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Output

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