☆☆思路：棧 + 頭插法。本題要求不能對節點進行翻轉，那麼對於逆序處理，首先應該想到資料結構【棧】。

class Solution {
    public ListNode addTwoNumbers(ListNode l1, ListNode l2) {
        // 不能對節點進行翻轉，故用到資料結構【棧】
        Stack<Integer> stack1 = new Stack<>();
        Stack<Integer> stack2 = new Stack<>();
        while (l1 != null
 
) {
            stack1.push(l1.val);
            l1 = l1.next;
        }
        while (l2 != null) {
            stack2.push(l2.val);
            l2 = l2.next;
        }
        // 【頭插法】 得到 翻轉後的結果連結串列
        ListNode head = null;
        int carry = 0;
        while (!stack1.isEmpty() || !stack2.isEmpty()) {
            
 
int x = stack1.size() == 0 ? 0 : stack1.pop();
            int y = stack2.size() == 0 ? 0 : stack2.pop();
            int temp = x + y + carry;
            carry = temp / 10;
            ListNode node = new ListNode(temp % 10);
            node.next = head;
            head = node;
        }
        if (carry == 1) {
            ListNode node 
 
= new ListNode(carry);
            node.next = head;
            head = node;
        }
        return head;
    }
}