求单链表的中间节点，用快慢指针

Node* findMid(Node* &head)
{
    if(head == NULL||head->next == NULL)
        return head;
    
    Node* p = head;
    Node* q = head;
    while(q->next->next&&(q = q->next))
    {
        p = p-> next;
        q = q->next;
    }
    return p;
}