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

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> s1 = new Stack<Integer>();
        Stack<Integer> s2 = new Stack<Integer>();
        while(l1 != null){
            s1.push(l1.val);
            l1 = l1.next;
        }
        while(l2 != null){
            s2.push(l2.val);
            l2 = l2.next;
        }
        int carry = 0, k1 = 0, k2 = 0;
        ListNode pre = null;
        while(!s1.isEmpty() || !s2.isEmpty() || carry != 0){
            k1 = s1.isEmpty() ? 0 : s1.pop();
            k2 = s2.isEmpty() ? 0 : s2.pop();
            int newval = (k1 + k2 + carry) % 10;
            carry = (k1 + k2 + carry) / 10;
            ListNode node = new ListNode(newval);
            node.next = pre;
            pre = node;
        }
        return pre;
    }