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.

Example
Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode ptr = null;
        ListNode output = null;
        int carry = 0;
        while (l1 != null || l2 != null) {
            int val1 = l1 != null? l1.val: 0;
            int val2 = l2 != null? l2.val: 0;
            int sum = carry + val1 + val2;
            carry = sum / 10;
            
            if (output == null) {
                output = new ListNode(sum % 10);
                ptr = output;
            } else {
                ptr.next = new ListNode(sum % 10);
                ptr = ptr.next;
            }
            if (l1 != null) l1=l1.next;
            if (l2 != null) l2=l2.next;
        }
        if (carry != 0) {
            ptr.next = new ListNode(carry);
        }
        return output;
    }
}