2.Add Two Numbers-两个单链表相加

You are given two linked lists representing two non-negative numbers. 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.

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

 /**
  * 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;//定义两个指针变量 c1 c2
         ListNode c2 = l2;
         ListNode head = new ListNode(0);//定义一个新链表的头结点
         ListNode d = head;//新链表的指针变量
         int sum = 0;
         while(c1 != null || c2 != null) {
             if(c1 != null) {
                 sum = sum + c1.val;
                 c1 = c1.next;
             }
             if(c2 != null) {
                 sum = sum + c2.val;
                 c2 = c2.next;
             }
             d.next = new ListNode(sum%10);
             d = d.next;
             sum = sum /10;
         }
         if(sum == 1) {
             d.next = new ListNode(1);
         }
         return head.next;

     }
 }