说来算是个基础题目，可是还是各种CE,各种WA，基础还是不扎实。
附上代码：
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 == NULL) return l2;
13 if (l2 == NULL) return l1;
14 // q: keep trace of the linked list that has been produced as the answer
15 // ll1: keep trace of l1
16 // ll2: keep trace of l2
17 ListNode *head(NULL), *q(NULL), *ll1 = l1, *ll2 = l2;
18 // c: holds the carry digit
19 int c = 0;
20 while (ll1!=NULL || ll2!=NULL) {
21 // sum: holds sum of ll1->val, ll2->val and c
22 int sum = 0;
23 if (ll1 != NULL) {
24 sum += ll1->val;
25 ll1 = ll1->next;
26 }
27 if (ll2 != NULL) {
28 sum += ll2->val;
29 ll2 = ll2->next;
30 }
31 sum += c;
32 ListNode *p = new ListNode(sum%10);
33 c = sum >= 10 ? 1 : 0;
34 if (head == NULL) {
35 head = q = p;
36 } else {
37 q->next = p;
38 q = p;
39 }
40 }
41 if (c) {
42 q->next = new ListNode(c);
43 }
44 return head;
45 }
46 };