Add Two Numbers

题目地址：
https://leetcode.com/problems/add-two-numbers/description/

题目：

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

解题思路：
这道题就是双指针，但是需要注意while的判断条件。

代码：

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
    ListNode c1 = l1;
    ListNode c2 = l2;
    ListNode sentinel = new ListNode(0);
    ListNode d = sentinel;
    int sum = 0;
    while (c1 != null || c2 != null) {
        sum /= 10;
        if (c1 != null) {
            sum += c1.val;
            c1 = c1.next;
        }
        if (c2 != null) {
            sum += c2.val;
            c2 = c2.next;
        }
        d.next = new ListNode(sum % 10);
        d = d.next;
    }
    if (sum / 10 == 1)
        d.next = new ListNode(1);
    return sentinel.next;
}