[LeetCode] 445. Add Two Numbers II

Problem
You are given two non-empty linked lists representing two non-negative integers. The most significant digit comes first 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.
Follow up:What if you cannot modify the input lists? In other words, reversing the lists is not allowed.
Example:
Input: (7 -> 2 -> 4 -> 3) + (5 -> 6 -> 4)Output: 7 -> 8 -> 0 -> 7
Solution

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        l1 = reverse(l1);
        l2 = reverse(l2);
        //add
        ListNode dummy = new ListNode(0), l3 = dummy;
        int carry = 0;
        while (l1 != null || l2 != null) {
            int sum = carry;
            if (l1 != null) {
                sum += l1.val;
                l1 = l1.next;
            }
            if (l2 != null) {
                sum += l2.val;
                l2 = l2.next;
            }
            l3.next = new ListNode(sum%10);
            l3 = l3.next;
            carry = sum/10;
        }
        if (carry != 0) l3.next = new ListNode(carry);
        return reverse(dummy.next);
    }
    private ListNode reverse(ListNode head) {
        ListNode pre = null;
        while (head != null) {
            ListNode next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
}