Solution.java

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        int sum = 0;
        ListNode res;
        if(l1 == null && l2 != null){
            l1 = new ListNode(0);
        } else if(l1 != null && l2 == null){
            l2= new ListNode(0);
        }else if(l1 == null && l2 == null){
            return null;
        }
        sum = sum + l1.val + l2.val;
        res = new ListNode(sum%10);          
        if(sum >= 10){
            if (l2.next!=null) {
                l2.next.val += 1;
            } else if (l1.next!=null) {
                l1.next.val+=1;
            }
            else{
                l1.next = new ListNode(0);
                l1.next.val +=1;
            }
            
        }
        res.next = addTwoNumbers(l1.next, l2.next);
        return res;
        
    }
}