Leetcode 2. Add Two Numbers

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

分析：两个链表中包含的数不一定一样多！！

 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         int flag = 0, add;
13         //创建一个节点要开辟空间，不能只定义一个指针，否则没有指向
14         ListNode* head = new ListNode(0), *p, *q, *l3;
15         p = l1;
16         q = l2;
17         l3 = head;
18         while(p != NULL && q != NULL){
19             l3->next = new ListNode((p->val + q->val + flag) % 10);
20             flag = (p->val + q->val  + flag) / 10;
21             l3 = l3->next;
22             p = p->next;
23             q = q->next;
24         }
25         while(p != NULL){
26             l3->next = new ListNode((p->val + flag) % 10);
27             flag = (p->val + flag) / 10;
28             l3 = l3->next;
29             p = p->next;
30         }
31          while(q != NULL){
32             l3->next = new ListNode((q->val + flag) % 10);
33             flag = (q->val + flag) / 10;
34             l3 = l3->next;
35             q = q->next;
36         }
37         if(flag == 1){//加入两个链表【9,9】，【8,9】，可知最后两个数加起来为19，需要在再创建一个节点最后放置1
38             l3->next = new ListNode(flag);
39             l3 = l3->next;
40         }
41         return head->next;
42         
43     }
44 };