023_linkedlistcycles.md

Linked List Cycles

[Date]

You have a singly-linked list and want to check if it contains a cycle.

A singly-linked list is built with nodes, where each node has:

• node.next — the next node in the list
• node.value - the data held in the node (e.g. if our linked list stores people in line at the movies, node.value might be the person's name)

For example:

class LinkedListNode
  attr_accessor :value, :next

  def initialize(value)
    @value = value
    @next  = nil
  end
end

A cycle occurs when a node’s @next points back to a previous node in the list. The linked list is no longer linear with a beginning and end — instead, it cycles through a loop of nodes.

Write a method contains_cycle() that takes the first node in a singly-linked list and returns a boolean indicating whether the list contains a cycle.

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Notes: A cycle can occur in the middle of a list, or it can simply mean the last node links back to the first node. Does your method work for both?