2. Add Two Numbers

 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         ListNode* newHead = new ListNode(0);
13         ListNode* pNode = newHead;
14         
15         int num = 0;
16         
17         while(l1 != NULL || l2 != NULL){
18             int val = num + (l1 ? l1->val : 0) + (l2 ? l2->val: 0);
19             num = val /10;
20             val = val % 10;
21             pNode->next = new ListNode(val);
22             pNode = pNode->next;
23             if(l1 != NULL){
24                 l1 = l1->next;
25             }
26             if(l2 != NULL){
27                 l2 = l2->next;
28             }
29         }
30         if(num != 0){
31             pNode->next = new ListNode(num);
32             pNode = pNode->next;
33         }
34         return newHead->next;
35     }
36 };