Leetcode: Add Two Numbers

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

Note the carry case, and cases when nodes are null.

/**
* Definition for singly-linked list.
* public class ListNode {
* int val;
* ListNode next;
* ListNode(int x) {
* val = x;
* next = null;
* }
* }
*/
public class Solution {
public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
if (l1 == null && l2 == null) {
return null;
}

ListNode res = new ListNode(0);
ListNode head = new ListNode(0);
res.next = head;
head = res;

int carry = 0;
while (l1 != null || l2 != null) {
int sum = (l1 == null ? 0 : l1.val) + (l2 == null ? 0 : l2.val) + carry;
head.next = new ListNode(sum % 10);
carry = sum / 10;
l1 = (l1 == null ? null : l1.next);
l2 = (l2 == null ? null : l2.next);
head = head.next;
}
if (carry != 0) {
head.next = new ListNode(carry);
}

return res.next;
}
}