Add Two Numbers

今天的题好难呀
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.
翻译：
您将得到两个非空链表，它们表示两个非负整数。这些数字以相反的顺序存储，它们的每个节点都包含一个数字。将这两个数字相加并以链表的形式返回。
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.

解析：

Screen Shot 2018-12-24 at 10.38.31 PM.png Screen Shot 2018-12-24 at 10.38.36 PM.png

难点在于如何写单链表

答案：

# Definition for singly-linked list.
# class ListNode:
#     def __init__(self, x):
#         self.val = x
#         self.next = None

class Solution:
    def addTwoNumbers(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        if l1 == None:
            return l2
        elif l2 == None:
            return l1
        
        remain = 0
        l = ListNode(None)
        result = l
        
        while l1 != None or l2 != None:
            if l1 == None:
                l1 = ListNode(0)
            elif l2 == None:
                l2 = ListNode(0)
            total = l1.val + l2.val + remain
            if total >=10 :
                total = total - 10
                remain =1
            else:
                remain = 0
            l.next = ListNode(total)
            l1 = l1.next
            l2 = l2.next
            l = l.next
            
        if remain == 1:
            l.next = ListNode(remain)
            
        return result.next