Add Two Numbers

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> list1 = new Stack<>();
        while(l1 != null) {
            list1.push(l1.val);
            l1 = l1.next;
        }
        
        Stack<Integer> list2 = new Stack<>();
        while(l2 != null) {
            list2.push(l2.val);
            l2 = l2.next;
        }
        
        ListNode current = new ListNode(-1);
        int carry = 0;
        while(!list1.isEmpty() || !list2.isEmpty() || carry == 1) {
            int val1 = 0, val2 = 0;
            if (!list1.isEmpty()) {
                val1 = list1.peek();
                list1.pop();
            }
            if (!list2.isEmpty()) {
                val2 = list2.peek();
                list2.pop();
            }
            
            int tempVal = val1 + val2 + carry;
            carry = tempVal / 10;
            
            if (current.val == -1) {
                current.val = tempVal % 10;
            } else {
                ListNode temp = new ListNode(tempVal % 10);
                temp.next = current;
                current = temp;
            }
        }
        return current;
    }
}