具体思想：

直接指针操作，没啥可说的；

具体代码：

/**
 * 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* oddEvenList(ListNode* head) {
        ListNode* p = head;
        ListNode* pre = head;
        ListNode* q = head;
        bool flag = false;
        if (!head || !head->next)
            return head;
        q = head->next;
        while (q != nullptr) {
            if (flag) {
                flag = false;
                ListNode* temp = p->next;
                pre->next = q->next;
                p->next = q;
                q->next = temp;
                p = p->next;
                q=pre->next;
            }
            else {
                flag = true;
                pre = pre->next;
                q = q->next;
            }
        }
        return head;
    }
};