isCircularLinkedList

Python

# A Circular Linked List is a linked list where the last node points back to an earlier node (usually the head) instead of null.
# Detect the start of a cycle in a circular linked list.
class Node:
  def __init__(self, value):
    self.value = value
    self.next = None

def has_cycle(head):
  if not head or not head.next:
    return False

  slow = fast = head

  while fast and fast.next:
    slow = slow.next
    fast = fast.next.next

    if slow == fast:
      return True

  return False

JavaScript

class Node {
  constructor(val) {
    this.val = val;
    this.next = null;
  }
}

/**
 * Detects if a linked list has a cycle. 
 * Detecting a cycle in a linked list involves using two pointers,
 * one moving at a slower pace than the other. 
 * If there is a cycle, the faster pointer will eventually catch up 
 * to the slower pointer.
 * @param {*} head 
 * @returns 
 */
export default function hasCycle(head) {
  if (!head || !head.next) {
    return false;
  }

  let slow = head;
  let fast = head;

  while (fast && fast.next) {
    slow = slow.next;
    fast = fast.next.next;

    if (slow === fast) {
      return true;
    }
  }

  return false;
}