Detecting Linked List Loops

In a circular linked list, we do NOT have a tail node. The "last node" points to a node within the list.

The current methods for iterating over a linked list such as for...of or forEach will not work with a circular linked list because these use a node that points to null to complete the loop. In a circular linked list, no node points to null.

Examples:

const l = new List()
const a = new Node('a')
const b = new Node('b')
const c = new Node('c')
l.head = a
a.next = b
b.next = c
c.next = b
circular(l) // true

Strategy - Circular Linked List

• use two pointers, slow and fast to iterate through the linked list.
• initialize both points to the head of the linked list.
• if the next two nodes after fast are defined, we will move fast two nodes ahead and slow one node ahead.
• we then do a check to see if fast is pointing to the same node as slow.
  • if it is, we know that the linked list is circular.
  • If not, we continue to iterate through the linked list, moving fast up two nodes and slow one node.

Circular Linked List Solution

function circular(list) {
  let slow = list.head
  let fast = list.head

  // loop as long as fast has two nodes ahead of it.
  while (fast.next && fast.next.next) {
    slow = slow.next
    fast = fast.next.next

    // compare slow and fast nodes
    // compare the nodes in memory, not the data
    if (slow === fast) {
      return true // circular
    }
  }
  return false // not circular
}