2. Add Two Numbers

---

You are given two non-empty linked lists representing two non-negative integers. 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.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

这个题目就是求和，但是要考虑进位问题。

/**
 * 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 head(0);
        ListNode* pre = &head;
        if(!l1)return l2;
        if(!l2)return l1;
        int extra = 0;
        while(l1||l2||extra){
            if(l1){
                extra += l1->val;
                l1 = l1->next;
            }
            if(l2){
                extra += l2->val;
                l2 = l2->next;
            }
            pre->next = new ListNode(extra%10);
            extra = extra/10;
            pre = pre->next;
        }
        return head.next;
    }
};