leetcode2: addTwoSum

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.
给你两个非空链表，表示两个非负整数。 数字以相反的顺序存储，每个节点都包含一个数字。 添加这两个数字并将其作为链接列表返回。

您可以假设这两个数字不包含任何前导零，除了数字0本身。
Example

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

class ListNode:
    def __init__(self,x):
        self.val = x
        self.next = none

class solution:
    def addTwoSum(self,l1,l2):
        a,b,carry = 0
        head = None
        last_node = None
        while l1 or l2:
            if l1:
                a = l1.val
                l1 = l1.next
            if l2:
                b = l2.val
                l2 = l2.next
            sum = a+b+carry
            carry,digit = divmod(sum,10)
            new_node = ListNode(digit)
            if not head:
                head = new_node
            else:
                last_node.next = new_node
            last_node = new_node
            a = b = 0
        if carry!=0:
            new_node = ListNode(carry)
            last_node.next = new_node
            last_node = new_node
        return head