21 merge two sorted lists

def (self, l1, l2):
  """
  :type l1: ListNode
  :type l2: ListNode
  :rtype: ListNode
  """     
  fs = ListNode(0)
  fp = fs
  while l1 != None and l2 != None:
    if l1.val < l2.val:
      fp.next = l1
      l1 = l1.next
    else:
      fp.next = l2
      l2 = l2.next
    fp = fp.next
  fp.next = l1 if l1 else l2
  return fs.next