[JS] Check if a linked list has a cycle

checkCycleInLikedList.js

// Using strategy of slower and faster pointers

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

function hasCycle(head) {
  if(!head) return false;
  
  let slower = head;
  let faster = head.next;
  while(faster) {
    if(slower === faster) return true;
    if(!faster.next) return false;
    slower = slower.next;      
    faster = faster.next.next;    
  }
  return false;
}


// ------------------------------------------- Test ---------------------------------------------------//
// null
console.log(hasCycle(null));  // false

// head
const head = new Node(); 
console.log(hasCycle(head));  // false

// head -> head
head.next = head;
console.log(hasCycle(head));  // true

// head -> node1
const node1 = new Node();
head.next = node1;
console.log(hasCycle(head));  // false

// head -> node1 -> node2
const node2 = new Node();
node1.next = node2;
console.log(hasCycle(head));  // false

// head -> node1 -> node2 -> node1
node2.next = node1;
console.log(hasCycle(head));  // true