题目链接

　　题目要求：

　　You are given two linked lists representing two non-negative numbers. 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.

　　Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
　　Output: 7 -> 0 -> 8

　　这道题难度不大，具体程序如下：

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     ListNode *next;
 6  *     ListNode(int x) : val(x), next(NULL) {}
 7  * };
 8  */
 9 class Solution {
10 public:
11     ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
12         if(!l1)
13             return l2;
14         else if(!l2)
15             return l1;
16         
17         ListNode *dummy = new ListNode(0);
18         ListNode *head = nullptr, *start = dummy;
19         int carry = 0;
20         while(l1 || l2)
21         {
22             if(l1 && l2)
23             {
24                 int sum = l1->val + l2->val + carry;
25                 l1->val = sum % 10;
26                 carry = sum / 10;
27                 start->next = l1;
28                 l1 = l1->next;
29                 l2 = l2->next;
30             }
31             else if(l1)
32             {
33                 int sum = l1->val + carry;
34                 l1->val = sum % 10;
35                 carry = sum / 10;
36                 start->next = l1;
37                 l1 = l1->next;
38             }
39             else
40             {
41                 int sum = l2->val + carry;
42                 l2->val = sum % 10;
43                 carry = sum / 10;
44                 start->next = l2;
45                 l2 = l2->next;
46             }
47             
48             start = start->next;
49         }
50         
51         if(carry)
52         {
53             ListNode *node = new ListNode(1);
54             start->next = node;
55         }
56         
57         head = dummy->next;
58         delete dummy;
59         dummy = nullptr;
60         
61         return head;
62     }
63 };