445. Add Two Numbers II

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

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<ListNode> stack1 = new Stack<>();
        Stack<ListNode> stack2 = new Stack<>();
        ListNode answer = null;
        while(l1 != null){
            stack1.push(l1);
            l1 = l1.next;
        }
        while(l2 != null){
            stack2.push(l2);
            l2 = l2.next;
        }
        int a = 0, b = 0, carry = 0, left = 0;
        while(!stack1.isEmpty() || !stack2.isEmpty()){
            if(!stack1.isEmpty())   {a = stack1.pop().val;}
            else    {a = 0;}
            if(!stack2.isEmpty())   {b = stack2.pop().val;}
            else    {b = 0;}
            left = (a + b + carry) % 10;
            carry = (a + b + carry) / 10;
            ListNode p = new ListNode(left);
            p.next = answer;
            answer = p;
        }
        if(carry != 0){
            ListNode p = new ListNode(carry);
            p.next = answer;
            answer = p;
        }
        return answer;
    }
}