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.

Example

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

題意：給定兩個非空鏈表來表示兩個非負整數。 數字以相反的順序存儲，每個節點包含一個數字。 對這兩個數字加和並將其以鏈表形式返回。假定這兩個數字不包含任何前導零，除了數字0本身。

思路：若存在空鏈表，直接返回另一個即可。若均不為空，就對應位置逐個相加。註意，提前實例化一個存放結果鏈表的頭結點。

 1 class ListNode:
 2     def __init__(self,x):
 
 3         self.val=x
 4         self.next=None
 5 
 6 class Solution:
 7     def addTwoNumbers(self,l1,l2):
 8         if l1 is None:  #若存在空鏈表，直接返回另一個即可
 9             return l2
10         elif l2 is None:
11             return l1
12         else:
13             carry = 0   #進位標誌
14             pNode = ListNode(0)  #
 
存放結果的結點數值先放0
15             pHead = pNode        #指向第一個元素的指針
16             while(l1 or l2):  #直到某個表為空
17                 sum = 0  #移動指針後先清空
18                 if(l1):
19                     sum = l1.val  #sum存下l1當下指向的數
20                     l1 = l1.next  #存數後移動指針
21                 if(l2):
22                     sum += l2.val  #sum進行對應位置加法運算
23                     l2 = l2.next   #存數後移動指針
24                 sum += carry   #加上進位
25                 pHead.next = ListNode(sum%10)  #存下當前位的數字
26                 pHead = pHead.next  #指向該數字
27                 carry = (sum>=10)   #表示進位
28             if(carry):   #若結束l1和l2的遍歷仍有進位
29                 pHead.next = ListNode(1)   #記下進位
30             pHead = pNode.next  #指向第一個位置
31             del pNode    #刪除頭結點
32             return pHead
33 
34 
35 if __name__==‘__main__‘:
36     l1_1 = ListNode(2)
37     l1_2 = ListNode(4)
38     l1_3 = ListNode(3)
39     l1_1.next = l1_2
40     l1_2.next = l1_3
41     l2_1 = ListNode(5)
42     l2_2 = ListNode(6)
43     l2_3 = ListNode(4)
44     l2_1.next = l2_2
45     l2_2.next = l2_3
46     l3_1=Solution().addTwoNumbers(l1_1,l2_1)
47     while l3_1 != None:
48         print(l3_1.val)
49         l3_1 = l3_1.next

leetcode:2-Add Two Numbers