注意点：

1. 最后的进位
2. (l1 == null || l1.next == null)
3. break;

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {

        boolean j = false;

        ListNode p = new ListNode(0);

        ListNode res = p;

        while(true){

            int a = (l1 != null ? l1.val : 0) + (l2 != null ? l2.val : 0) + (j ? 1 : 0);

            if(a >= 10){

                j = true;

                p.val = a - 10;

            }else{

                j = false;

                p.val = a;

            }

            l1 = (l1 == null || l1.next == null) ? null : l1.next;

            l2 = (l2 == null || l2.next == null) ? null : l2.next;

            boolean ok = l1 == null && l2 == null;

            if(! ok){

                p.next = new ListNode(0);

                p = p.next;

            }else

                break;

        }

        if(j)

            p.next = new ListNode(1);

        return res;

    }