@startuml
title Step 0 — Initial State

object "Node 1" as n1 {
  value = 1
}

object "Node 2" as n2 {
  value = 2
}

object "Node 3" as n3 {
  value = 3
}

n1 --> n2 : next
n2 --> n3 : next
n3 --> "null" : next

object "previous" as prev
object "current" as cur

prev --> "null"
cur --> n1

note bottom
Before the loop starts:

previous = null
current  = head

The original list is still intact.
end note

@enduml