题：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
自己写了段30多行，分了许多条件，看了眼讨论里别人的11行代码...自己实在是太菜了，在这里贴一下这个11行代码。
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; }