答案

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
 public ListNode addTwoNumbers(ListNode l1, ListNode l2)
    {
        ListNode head = new ListNode(0);
        ListNode last = head;
        int carry = 0;
        int current = 0;
        while (l1 != null || l2 != null || carry > 0)
        {
            current = carry;
            if (l1 != null)
            {
                current += l1.val;
                l1 = l1.next;
            }
            if (l2 != null)
            {
                current += l2.val;
                l2 = l2.next;
            }
            carry = current / 10;
            current %= 10;
            ListNode p = new ListNode(current);
            last.next = p;
            last = p;
        }
        return head.next;
    }
}