Leetcode #2 Add two number

Q: 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

keys: 1. 使用dummy node记录head. 

2. 对于两个list的操作，一般使用 while l1 or l2， if l1, if l2 来处理两个list不一样长的情况。

 class Solution(object):
     def addTwoNumbers(self, l1, l2):
         """
         :type l1: ListNode
         :type l2: ListNode
         :rtype: ListNode
         """
         dummy = cur = ListNode(0)
         carry = 0

         while l1 or l2:
             a = l1.val if l1 else 0
             b = l2.val if l2 else 0

             sum = a+b+carry
             cur.next = ListNode(sum%10)
             carry = sum/10
             cur = cur.next

             if l1:
                 l1 = l1.next
             if l2:
                 l2 = l2.next

         if carry > 0:
             cur.next = ListNode(1)

         return dummy.next