解题思路

使用双栈

 public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        Stack<Integer> stack1 = new Stack<>();
        Stack<Integer> stack2 = new Stack<>();
        ListNode node1 = l1;
        while (node1 != null){
            stack1.push(node1.val);
            node1 = node1.next;
        }
        ListNode node2 = l2;
        while (node2 != null){
            stack2.push(node2.val);
            node2 = node2.next;
        }
        ListNode head = null;
        int flag = 0;
        while(!stack1.isEmpty() || !stack2.isEmpty() || flag != 0){
            int value = flag;
            if (!stack1.isEmpty())
                value += stack1.pop();
            if (!stack2.isEmpty())
                value += stack2.pop();
            ListNode node = new ListNode(value % 10);
            flag = value / 10;
            node.next = head;
            head = node;
        }
        return head;
    }