0

I am trying to detect a cycle in a linked list I created (I'm practicing for interviews questions). I understand the logic involved in Floyd's tortoise-hare algorithm but the function is always returning false...

Here's my linked list: 

class LinkedList {
  constructor() {
    this.length = 0;
    this.head = null;
  }
  insert(index, value) {
    if (index < 0 || index > this.length) {
       throw new Error("Index error");
    }
    const newNode = {
       value
    };
    if (index == 0) {
      newNode.next = this.head;
      this.head = newNode;
    } else {
      const node = this._find(index - 1);
      newNode.next = node.next;
      node.next = newNode;
    }
    this.length++;
  }
  ...
}

//Inserting values
const linkedList = new LinkedList();
linkedList.insert(0, 12);
linkedList.insert(1, 24);
linkedList.insert(2, 65);
linkedList.insert(3, 23);
linkedList.insert(4, 9);
linkedList.insert(5, 7);
linkedList.insert(6, 13);
linkedList.insert(7, 65);
linkedList.insert(8, 20);

Here is my cycle-detection function, it returns false even if there is a cycle:

 function containsCycle(firstNode) {
   // Start both at beginning
   let slow = firstNode;
   let fast = firstNode;
   // Until end of the list
   while (fast && fast.next) {
     slow = slow.next;
     fast = fast.next.next;
   // fast is about to "lap" slow
     if (fast === slow) {
       return true;
     }
    }
   // fast hit the end of the list
   return false;
 }

//Calling function
containsCycle(linkedList.head);

I just cannot find what's wrong with my function and the more I try to figure it out the more narrow minded I become... any guidance would be very much appreciated!

Jack Surtees
  • 39
  • 1
  • 8
  • Please provide a [mcve] with a graph that actually contains a cycle. – Bergi Apr 23 '19 at 19:12
  • I just read the guidelines for a Minimal, Complete, and Verifiable example and I see why you asked me to provide such an example. Quick question before I do however: my list doesn’t actually contain a cycle? – Jack Surtees Apr 23 '19 at 19:52
  • 2
    No, your list doesn't contain a cycle. It is a chain having 9 different nodes, and the last one does not have a next one linked to it. – trincot Apr 23 '19 at 19:55
  • 1
    @JackSurtees A linked *list* never has any cycles, as a list is characterised by being linear and having an end. Repeating elements doesn't create a cycle - a cycle would lead to an infinite list. Your `insert` method never creates such a structure. – Bergi Apr 23 '19 at 19:56
  • Bergi and trincot, many thanks. You cleared up what was a BIG understanding for me – Jack Surtees Apr 23 '19 at 20:04

1 Answers1

1

You're creating new nodes each insertion. E.g. you're 3rd and 8th nodes both have values 65, but are not equal (they're different objects, so the === condition will fail). More importantly, they have different .next nodes and your slwo and fast iterators will not loop back to the 4th element after traversing the 8th element.

junvar
  • 11,151
  • 2
  • 30
  • 46