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;}
};