*Question: Write a function testListCycle() that takes node as argument for a given singly-linked list and returns a boolean indicating whether the list contains a cycle.*

A **cycle** meaning when a node’s next points *back to a previous node in the list* which makes list non-linear and it will be loop.

*To solve this problem first we need to introduce what is singly-linked list, and how to access the node and steps to check loop!*

A singly linked list is collection of nodes wherein each node has 2 parts: data and a pointer to the next node. The list terminates with a node pointing at `null`

.

Main operations on a linked list are: *insert and delete.*

Here is the class which creates singly linked list.

class Node{ constructor(data, next = null){ this.data = data, this.next = next } } class LinkedList{ constructor(){ this.head = null; } } let list = new LinkedList();

Let’s talk about the algorithm & data structure we can use to solve this problem!

We can use Set data structure to store values for each node.

*Algorithm:*

- If the current node is already in our set, we have a cycle. Return true.
- If the current node is null we’ve hit the end of the list. Return false.
- Else throw the current node in our set and keep going.

*Here is the Working Solution:*

```
cost testListCycle = (node) => {
let curPointer = firstNode;
let nextPointer = firstNode;
while (curPointer && nextPointer.next) {
curPointer = curPointer.next;
nextPointer = nextPointer.next.next;
if (nextPointer === curPointer) {
return true;
}
}
return false;
}
```