2. Add Two Numbers

    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if(l1 == null) return l2;
        if(l2 == null) return l1;
        
        ListNode head = new ListNode(0);
        ListNode l3 = head;
        
        int carry = 0;
        
        while(l1 != null || l2 !=null || carry != 0){
            if(l1 != null) {
                carry += l1.val;
                l1=l1.next;
            }
            if(l2 != null) {
                carry += l2.val;
                l2=l2.next;
            }
            l3.next = new ListNode(carry%10);
            l3 = l3.next;
            carry /= 10;
        }
        
        return head.next;

    }