/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteMiddle(ListNode* head) {
        if (!head) return nullptr;
        if (!head->next) return nullptr;
        if (!head->next->next) {
            head->next = nullptr;
            return head;
        }
        ListNode *p1 = head->next->next, *p2 = head;
        while (p1) {
            p1 = p1->next;
            if (p1) p1 = p1->next;
            else break;
            p2 = p2->next;
        }
        p2->next = p2->next->next;
        return head;
    }
};