struct ListNode* middleNode(struct ListNode* head){
    if (head == NULL || head->next == NULL) {
        return head;
    }
    struct ListNode* fast = head, * slow = head;
    // fast->next != NULL && fast->next->next != NULL 返回的是第一个中间节点
    while (fast!=NULL && fast->next != NULL) {
        slow = slow->next;
        fast = fast->next->next;
    }
    return slow;
}