Add Two Numbers（from leetcode python 链表）

给定两个非空链表来表示两个非负整数。位数按照逆序方式存储，它们的每个节点只存储单个数字。将两数相加返回一个新的链表。

你可以假设除了数字 0 之外，这两个数字都不会以零开头。

示例：

输入：(2 -> 4 -> 3) + (5 -> 6 -> 4)
输出：7 -> 0 -> 8
原因：342 + 465 = 807

　　

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

 class Solution(object):
     def addTwoNumbers(self, l1, l2):
         """
         :type l1: ListNode
         :type l2: ListNode
         :rtype: ListNode
         """
         if l1 == None: return l2
         if l2 == None: return l1
         // 使用dummyHead指向头结点，最后返回dummyHead.next
         dummy_head = ListNode(0)
         // 指针
         pt = dummy_head
         // 是否进位
         carry = 0
         while l1 and l2:
             pt.next = ListNode((l1.val + l2.val + carry) % 10)
             carry = (l1.val + l2.val + carry) / 10
             pt = pt.next;
             l1 = l1.next;
             l2 = l2.next
         if l1:
             while l1:
                 pt.next = ListNode((l1.val + carry) % 10)
                 carry = (l1.val + carry) / 10
                 pt = pt.next;
                 l1 = l1.next
             if carry == 1:
                 pt.next = ListNode(1)
             return dummy_head.next
         if l2:
             while l2:
                 pt.next = ListNode((l2.val + carry) % 10)
                 carry = (l2.val + carry) / 10
                 pt = pt.next;
                 l2 = l2.next
             if carry == 1:
                 pt.next = ListNode(1)
             return dummy_head.next
         if carry == 1:
             pt.next = ListNode(1)
         return dummy_head.next