2. Add Two Numbers

https://leetcode.com/problems/add-two-numbers/

solution:
需要考虑进位的问题。 然后就是linked list细节了： 生成新节点等等。

代码：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* addTwoNumbers(ListNode* l1, ListNode* l2) {
        ListNode dummy(-1);
        ListNode* p = &dummy;
        int carrier = 0;
        while(l1||l2){
            if(l1) {
                carrier += l1->val;
                l1 = l1->next;
            }
            if(l2) {
                carrier += l2->val;
                l2 = l2->next;
            }
            p->next = new ListNode(carrier%10);
            p = p->next;
            carrier /= 10;
        }
        if(carrier){
            p->next = new ListNode(carrier);
            p = p->next;
        }
        return dummy.next;
   };