2. Add Two Numbers

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

看到这个题目我首先就想到了大二学习数字逻辑的加法器原理，结果中每一位的数值都由两个加数中对应位和上一位的进位决定。
另外做这个题时要设立两个节点，一个要一直指向头部，返回时使用。另一个指向尾部，以便添加新的元素。

# Definition for singly-linked list.# class ListNode(object):#     def __init__(self, x):#         self.val = x#         self.next = Noneclass Solution(object):    def addTwoNumbers(self, l1, l2):        """        :type l1: ListNode        :type l2: ListNode        :rtype: ListNode        """        l3 = tmp = ListNode(0)        carry = 0        while l1 or l2 or carry:            v1 = v2 = 0            if l1:                v1 = l1.val                l1 = l1.next            if l2:                v2 = l2.val                l2 = l2.next            tmp.next = ListNode((v1 + v2 + carry)%10)            tmp = tmp.next            carry = (v1 + v2 + carry)/10        return l3.next