new_contents: # -*- coding:utf-8 -*-
  # 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):
          overflow = 0
          head = ListNode(0)
          node = head
          while l1 is not None or l2 is not None:
              node.next = ListNode(0)
              if l1 is None:
                  v1 = 0
              else:
                  v1 = l1.val
                  l1 = l1.next
              if l2 is None:
                  v2 = 0
              else:
                  v2 = l2.val
                  l2 = l2.next
              s = v1 + v2 + overflow
              node.next.val = s % 10
              overflow = s // 10
              node = node.next
          if overflow:
              node.next = ListNode(overflow)
          return head.next
  
subject: add leetcode Add Two Numbers