postApril 8, 2026

Detect a Cycle in a Linked List

#javascript#linked-list#two-pointers#algorithms

Medium⏱️ Logic Challenge

Implement hasCycle(head) for a singly linked list. Each node has {val, next}. Return true if the list contains a cycle. O(1) space required.

Ver solución

function hasCycle(head) {
  let slow = head;
  let fast = head;

  while (fast !== null && fast.next !== null) {
    slow = slow.next;          // move 1 step
    fast = fast.next.next;     // move 2 steps

    if (slow === fast) return true;  // cycle!
  }
  return false;
}

// Floyd's Cycle Detection (tortoise & hare):
// If there's a cycle, fast will eventually lap slow.
// If no cycle, fast reaches null first.
// O(n) time, O(1) space — no extra Set needed.