2. Add Two Numbers (C++)

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.

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8

答案：

/**
* 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 preHead(0), *p = &preHead;
　　　　int extra = 0;
　　　　while (l1 || l2 || extra) {
　　　　　　int sum = (l1 ? l1->val : 0) + (l2 ? l2->val : 0) + extra;
　　　　　　extra = sum / 10;
　　　　　　p->next = new ListNode(sum % 10);
　　　　　　p = p->next;
　　　　　　l1 = l1 ? l1->next : l1;
　　　　　　l2 = l2 ? l2->next : l2;
　　　　}
　　　　return preHead.next;
　　}
};