# Detect start of a loop in linked list

#### Problem :

Given a linked list, implement an algorithm which returns the node at the beginning of the loop.

This post is a follow-up of

– JavaScript Linked List Example

– Detect a loop in cyclic/circular linked list.

– Loop Length in cyclic/cicular linked list.

I recommend reading those posts first, as the following code uses the methods from it.

#### Logic:

- Find the loop, using the same logic Detect a loop in cyclic/circular linked list.
- Move
`p1`

at the head of the linked list, and keep`p2`

at the same location where`p1`

and`p2`

met. - Move
`p1`

,`p2`

one step at a time, when they will meet again it’s the beginning of the loop.

Watch the following video to understand the Floyd’s cycle-finding algorithm!

This solution is a follow-up of JavaScript Linked List Example. I recommend reading it first, as the following code uses the method from it.