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

Solution:

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Deque<Integer> st1 = new ArrayDeque();
        Deque<Integer> st2 = new ArrayDeque();
        while (l1 != null) {
            st1.push(l1.val);
            l1 = l1.next;
        }
        while (l2 != null) {
            st2.push(l2.val);
            l2 = l2.next;
        }
        ListNode prev = null;
        int carry = 0;
        while (!st1.isEmpty() && !st2.isEmpty()) {
            int a = st1.pop();
            int b = st2.pop();
            int val = (a + b + carry) % 10;
            carry = (a + b + carry) / 10;
            ListNode curr = new ListNode(val);
            curr.next = prev;
            prev = curr;
        }
        while (!st1.isEmpty()) {
            int a = st1.pop();
            int val = (a + carry) % 10;
            carry = (a + carry) / 10;
            ListNode curr = new ListNode(val);
            curr.next = prev;
            prev = curr;
        }
        while (!st2.isEmpty()) {
            int a = st2.pop();
            int val = (a + carry) % 10;
            carry = (a + carry) / 10;
            ListNode curr = new ListNode(val);
            curr.next = prev;
            prev = curr;
        }
        if (carry != 0) {
            ListNode curr = new ListNode(carry);
            curr.next = prev;
            prev = curr;
        }
        return prev;
    }
}