445. Add Two Numbers II(Java)

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

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {        Stack<Integer> s1 = new Stack<>();        Stack<Integer> s2 = new Stack<>();        while (l1 != null) {            s1.push(l1.val);            l1 = l1.next;        }        while (l2 != null) {            s2.push(l2.val);            l2 = l2.next;        }        int sum = 0;        ListNode list = new ListNode(0);        while (!s1.empty() || !s2.empty()) {            if (!s1.empty()) sum += s1.pop();            if (!s2.empty()) sum += s2.pop();            list.val = sum % 10;            ListNode head = new ListNode(sum / 10);            head.next = list;            list = head;            sum /= 10;        }        return list.val == 0 ? list.next : list;    }}