input: Write a python function to reverse a linked list.
  assert llist.head.data == 3
  assert llist.head.next.data == 2
  assert llist.head.next.next.data == 1
output: class Node:
      def __init__(self, data):
          self.data = data
          self.next = None
  
  class LinkedList:
      def __init__(self):
          self.head = None
  
      def push(self, new_data):
          new_node = Node(new_data)
          new_node.next = self.head
          self.head = new_node
  
      def reverse(self):
          prev = None
          current = self.head
          while current:
              next_node = current.next
              current.next = prev
              prev = current
              current = next_node
          self.head = prev