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Linked List Challenge 3

Hey,

I was solving Linked Lists Challenge 3 Linked List Challenges and I pretty much managed to write the exact solution minus one line. Without that one line the solution gets stuck, but I can’t figure out why. The accompanying text does not talk of its exact significance either.

mutating func reverse() {
  tail = head
  var prev = head
  var current = head?.next
  prev?.next = nil

  while current != nil {
  let next = current?.next
  current?.next = prev
  prev = current
  current = next
}
head = prev

}

I hadn’t included the following line

  prev?.next = nil

I know its eventually important, because the new tail shouldn’t have a next, but what I couldn’t understand was that why does the algorithm not run unless I add this line.