题目描述：
You are given two non-empty linked lists representing two non-negative integers. 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.
You may assume the two numbers do not contain any leading zero, except the number 0 itself.
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.
思路：
小学加法思路，判断是否需要进一，用left来做记录。用dummy来记录。（dummy其实只是一个node，然后把计算得到的node依次在其后记录，最终的dummy.next其实也是一个node）
代码：
def addTwoNumbers(self, l1, l2):
"""
:type l1: ListNode
:type l2: ListNode
:rtype: ListNode
"""
curr = dummy = ListNode(0)
left = 0
while l1 or l2 or left:
(l1, num1) = (l1.next, l1.val) if l1 else (None, 0)
(l2, num2) = (l2.next, l2.val) if l2 else (None, 0)
left, num = divmod(num1 + num2 + left, 10)
curr.next = ListNode(num)
curr = curr.next
return dummy.next
思路其实很简单，只是对Linked List有所认识就可以做出来。
