ListNode* middleNode(ListNode* head) {
        int count = 0;
        auto headp = head;
        while (head->next) {
            ++count;
            head = head->next;
        }
        int i = count % 2 ? 1 + count / 2 : count / 2;
        while(i --> 0) {
            headp = headp->next;
        }
        return headp;
    }