下面函数使用快慢指针查找单链表的中间结点。横线处应填写（ ）。

struct Node {
   int value;
   Node *next;
};

Node *middle(Node *head) {
   Node *slow = head;
   Node *fast = head;
   while (fast != nullptr && fast->next != nullptr) {
      slow = slow->next;
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   }
   return slow;
}

{{ select(1) }}

• fast = fast->next;
• fast = fast->next->next;
• fast = slow->next;
• fast = head->next;