new_contents: # Definition for singly-linked list.
  # class ListNode:
  #     def __init__(self, x):
  #         self.val = x
  #         self.next = None
  
  
  class Solution:
      # @return a ListNode
  
      def addTwoNumbers(self, l1, l2):
          return self.add(l1, l2, 0)
  
      def add(self, l1, l2, adder):
          if l1 is None and l2 is None and adder == 0:
              return None
  
          res = ListNode(adder)
          if l1:
              res.val += l1.val
          if l2:
              res.val += l2.val
          adder = res.val/10
          res.val %= 10
          res.next = self.add(
              l1.next if l1 else None,
              l2.next if l2 else None,
              adder)
          return res
  
subject: add solution Add Two Numbers