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）

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode head = new ListNode(0), p = head;
        int sum = 0;
        while (l1 != null || l2 != null) {
            sum /= 10;
            if (l1 != null) {
                sum += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                sum += l2.val;
                l2 = l2.next;
            }
            ListNode newNode = new ListNode(sum % 10);
            p.next = newNode;
            p = p.next;
        }
        if (sum / 10 == 1) p.next = new ListNode(1);
        return head.next;        
    }