input: Write a python function to reverse a linked list. 
  assert reverseLinkedList(Node(1))==Node(1)
  assert reverseLinkedList(Node(1, Node(2))).value==2
  assert reverseLinkedList(Node(1, Node(2, Node(3, Node(4))))).value==4
output: class Node:
      def __init__(self, value, next=None):
          self.value = value
          self.next = next
  
  def reverseLinkedList(head):
      prev = None
      current = head
      while current:
          next_node = current.next
          current.next = prev
          prev = current
          current = next_node
      return prev