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.

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

含义：给定两个列表，求加法（注意从高位开始）

 1     public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
 2         ListNode head = new ListNode(0), p = head;
 3         int sum = 0;
 4         while (l1 != null || l2 != null) {
 5             sum /= 10;
 6             if (l1 != null) {
 7                 sum += l1.val;
 8                 l1 = l1.next;
 9             }
10             if (l2 != null) {
11                 sum += l2.val;
12                 l2 = l2.next;
13             }
14             ListNode newNode = new ListNode(sum % 10);
15             p.next = newNode;
16             p = p.next;
17         }
18         if (sum / 10 == 1) p.next = new ListNode(1);
19         return head.next;        
20     }