解题思路：

缺位的节点进行补零处理，如973+23补充为973+023

注意相加的进位问题

class Solution {public ListNode addTwoNumbers(ListNode l1, ListNode l2) {ListNode head = null, tail = null;int carry = 0;while (l1 != null || l2 != null) {int n1 = l1 != null ? l1.val : 0;int n2 = l2 != null ? l2.val : 0;int sum = n1 + n2 + carry;if (head == null) {head = tail = new ListNode(sum % 10);} else {tail.next = new ListNode(sum % 10);tail = tail.next;}carry = sum / 10;if (l1 != null) l1 = l1.next;if (l2 != null) l2 = l2.next;}if (carry > 0) tail.next = new ListNode(carry);return head;}
}