[leetcode]445. Add Two Numbers II

public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        if (l1==null&&l2==null) return null;
        Stack<Integer> s1 = new Stack<>();
        Stack<Integer> s2 = new Stack<>();
        ListNode list = new ListNode(0);
        while (l1!=null)
        {
            s1.push(l1.val);
            l1 = l1.next;
        }
        while (l2!=null)
        {
            s2.push(l2.val);
            l2 = l2.next;
        }
        int sum = 0;
        while (!s1.isEmpty()||!s2.isEmpty()||sum!=0)
        {
            if (!s1.isEmpty()) sum+=s1.pop();
            if (!s2.isEmpty()) sum+=s2.pop();
            list.val = sum%10;
            ListNode h = new ListNode(0);
            h.next = list;
            list = h;
            sum /=10;
        }
        return list.next;
    }