Cannot reverse linked list, but want to add from the end of this linked list: Use a Stack.

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class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<ListNode> st1 = new Stack<>();
        Stack<ListNode> st2 = new Stack<>();
        while (l1 != null) {
            st1.push(l1);
            l1 = l1.next;
        }
        while (l2 != null) {
            st2.push(l2);
            l2 = l2.next;
        }
        ListNode res = new ListNode(0);
        int sum = 0;
        while (!st1.empty() || !st2.empty()) {
            if (!st1.empty()) sum += st1.pop().val;
            if (!st2.empty()) sum += st2.pop().val;
            res.val = sum % 10;
            ListNode head = new ListNode(sum / 10);
            head.next = res;
            res = head;
            sum /= 10;
        }
        return res.val == 0 ? res.next : res;
    }
}