LeetCode 2: Add Two Numbers

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) { val = x; }
 7  * }
 8  */
 9 public class Solution {
10     public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
11         if (l1 == null) {
12             return l2;
13         }
14         
15         if (l2 == null) {
16             return l1;
17         }
18         ListNode result = new ListNode(0);
19         ListNode tail = result;
20         int c = 0;
21         while (l1 != null || l2 != null) {
22             int n1 = 0;
23             int n2 = 0;
24             if (l1 != null) {
25                 n1 = l1.val;
26                 l1 = l1.next;
27             }
28             
29             if (l2 != null) {
30                 n2 = l2.val;
31                 l2 = l2.next;
32             }
33             tail.next = new ListNode((n1 + n2 + c)%10);
34             tail = tail.next;
35             c = (n1 + n2 + c) / 10;
36         }
37         
38         if (c > 0) {
39             tail.next = new ListNode(c);
40         }
41         
42         return result.next;
43     }
44 }