原题：
You are given two linked lists representing two non-negative numbers.
The digits are stored in reverse order and each of their nodes contain a single digit
Add the two numbers and return it as a linked list
Input: (2->4->3) + (5->6->4)
Output: 7->0->8
代码如下，递归调用add函数即可：
class Solution:
# @param {ListNode} l1
# @param{ListNode}l2
# return {ListNode} def addTwoNumbers(self, l1, l2):
return self.add(l1, l2)
def add(self, l1, l2, s=0):
l = ListNode(s)
if l1:
l.val += l1.val
l1 = l1.next
if l2:
l.val += l2.val
l2 = l2.next
s= l.val/10
l.val = l.val % 10
if l1 or l2 or s: l.next = self.add(l1, l2, s)
return l