Leetcode 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

Notice that "in reverse order"~~ so 465 + 342 = 807

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {               ListNode c1 = l1;//to protect the head node        ListNode c2 = l2;        ListNode sentinel = new ListNode(0);        ListNode d = sentinel;        int sum = 0;        while (c1 != null || c2 != null) {            sum /= 10;//from the low digit to the high digit            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)//the highest  1            d.next = new ListNode(1);        return sentinel.next;            }}