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

Solution:

/**
* 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) {
return addTwoNumbers(l1,l2,0);
}
private ListNode addTwoNumbers(ListNode l1, ListNode l2, int a){
if(l1 == null && a == 0) {
return l2;
}
else  if(l2 == null && a == 0) return l1;
else {
int y;
int p = ((l1 == null) ? 0 : l1.val);
int q = ((l2 == null) ? 0 : l2.val);
int temp = (p + q + a)%10;
y = (p + q + a)/10;
ListNode resListNode = new ListNode(temp);
resListNode.next = addTwoNumbers(l1==null? null: l1.next,l2==null? null:l2.next ,y);
return resListNode;
}
}
}