链表相加

https://leetcode-cn.com/problems/add-two-numbers/

这里的两个数已经翻转了，所以不需要进行反转直接相加即可
需要注意的是进位，在最后的时候如果l1, l2都没有数据，但是addition进位为1，还需要在最后加一个值为1的node

# Definition for singly-linked list.
# class ListNode(object):
#     def __init__(self, val=0, next=None):
#         self.val = val
#         self.next = next
class Solution(object):
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        head = tail = None
        addition = 0
        while (l1 or l2 or addition):
            v1 = l1.val if l1 else 0
            v2 = l2.val if l2 else 0
            s = v1 + v2 + addition
            value = s % 10
            addition = 1 if s > 9 else 0
            if not head:
                tail = head = ListNode(value)
            else:
                tmp = ListNode(value)
                tail.next = tmp
                tail = tail.next
            l1 = l1.next if l1 else None
            l2 = l2.next if l2 else None
        return head