思路：遍历链表生成奇链表和偶链表，然后拼接两个链表生成新的链表。

/**
 * 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* pre = new ListNode(-1);
        ListNode* pre_temp = pre;
        ListNode* end = new ListNode(-1);
        ListNode* end_temp = end;
        int i = 1;//用来判断奇偶
        while(head){
            if(i%2==1){
                pre_temp->next = head;
                pre_temp = pre_temp->next;
            }else{
                end_temp->next = head;
                end_temp = end_temp->next;
            }
            i++;
            head = head->next;
        }
        end_temp->next = nullptr;
        pre_temp->next = end->next;
        return pre->next;
    }
};