LeetCode: Add Two Numbers

/**
 * 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;
        }
    }
}