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class Solution
{
public:
    ListNode* removeDuplicateNodes(ListNode* head)
    {
        if (!head || !head->next)
            return head;  // 空链表或单节点链表直接返回

        ListNode *pre = head, *cur = head->next;
        std::unordered_set<int> hash;
        hash.insert(pre->val);  // 将头结点的值加入哈希集合

        while (cur)
        {
            if (hash.count(cur->val))  // 当前节点值已出现，跳过该节点
            {
                pre->next = cur->next;
                cur = pre->next;
            }
            else  // 新值，保留该节点
            {
                hash.insert(cur->val);
                pre = cur;
                cur = cur->next;
            }
        }

        return head;
    }
};