LeetCode 2. Add Two Numbers

You are given two non-empty linked lists representing two non-negative integers. The digits are stored in reverse order and each of their nodes contain a single digit. Add the two numbers and return it as a linked list.

You may assume the two numbers do not contain any leading zero, except the number 0 itself.

Example:

Input: (2 -> 4 -> 3) + (5 -> 6 -> 4)
Output: 7 -> 0 -> 8
Explanation: 342 + 465 = 807.

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        ListNode first = new ListNode(-1);//指向结果list的头结点
        ListNode iter = first;
        int addOne = 0;

        while (l1.val < 0) l1 = l1.next;
        while (l2.val < 0) l2 = l2.next;

        while (l1 != null && l2 != null) {
            int val = l1.val + l2.val + addOne;
            addOne = val / 10;//相加的进位
            val %= 10;
            ListNode listNode = new ListNode(val);
            listNode.next = null;
            iter.next = listNode;

            l1 = l1.next;
            l2 = l2.next;
            iter = iter.next;
        }
        if (addOne != 0 && l1 == null && l2 == null){
            ListNode listNode = new ListNode(addOne);
            listNode.next = null;
            iter.next = listNode;
        }

        if (l1 != null) {
            l1.val += addOne;
            iter.next = l1;
            while (l1.val / 10 > 0 && l1 != null) {
                addOne = l1.val / 10;
                l1.val %= 10;
                if (l1.next == null){
                    ListNode listNode = new ListNode(addOne);
                    listNode.next = null;
                    l1.next = listNode;
                    break;
                }
                l1 = l1.next;
                l1.val += addOne;
            }
        }

        if (l2 != null) {
            l2.val += addOne;
            iter.next = l2;

            while (l2.val / 10 > 0 && l2 != null) {
                addOne = l2.val / 10;
                l2.val %= 10;
                if (l2.next == null){
                    ListNode listNode = new ListNode(addOne);
                    listNode.next = null;
                    l2.next = listNode;
                    break;
                }
                l2 = l2.next;
                l2.val += addOne;
            }
        }

        // 输出结果
//        while (first != null) {
//            System.out.print(first.val + "->");
//            first = first.next;
//        }
        return first.next;//null
    }
}