leetcode 2 两数之和

　　　　　　ListNode root  = new ListNode(0);
            ListNode cur = root;
            int retain = 0;
            while (l1!=null || l2!=null || retain!=0) {
                int l1_val = l1==null ? 0 : l1.val;
                int l2_val = l2==null ? 0 : l2.val;
                int sum = l1_val+l2_val+retain;
                //cur.val = sum%10;
                cur.next = new ListNode(sum%10);
                cur = cur.next;
                retain = sum /10;

                if(l1!=null) l1 = l1.next;
                if(l2!=null) l2 = l2.next;
            }
            cur.next=null;
            return root.next;