Add Two Numbers

 1 /**
 2  * Definition for singly-linked list.
 3  * public class ListNode {
 4  *     int val;
 5  *     ListNode next;
 6  *     ListNode(int x) {
 7  *         val = x;
 8  *         next = null;      
 9  *     }
10  * }
11  */
12 public class Solution {
13     /**
14      * @param l1: the first list
15      * @param l2: the second list
16      * @return: the sum list of l1 and l2 
17      */
18     public ListNode addLists(ListNode l1, ListNode l2) {
19         
20         if (l1 == null) return l2;
21         if (l2 == null) return l1;
22         
23         int value1 = 0;
24         int value2 = 0;
25         int carry = 0;
26         ListNode head = new ListNode(0);
27         ListNode current = head;
28         
29         
30         while (l1 != null || l2 != null) {
31             value1 = (l1 == null ? 0 : l1.val);
32             value2 = (l2 == null ? 0 : l2.val);
33             int value = (value1 + value2 + carry) % 10;
34             
35             current.next = new ListNode(value);
36             current = current.next;
37             
38             carry = (value1 + value2 + carry) / 10;
39             l1 = (l1 == null) ? null : l1.next;
40             l2 = (l2 == null) ? null : l2.next;
41         }
42         if (carry == 1) {
43             current.next = new ListNode(carry);
44         }
45         return head.next;
46     }
47 }