剛剛開始沒想到，後來想到可以使用兩個表頭，分別串成兩個表，然後奇數連結串列後面接上偶數連結串列，結束。

/**
 * 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) 
    {
        if(!head || !head->next)
        return head;
        ListNode* even = head->next;
        ListNode* temp_odd = head;
        ListNode* temp_even = head->next;
        while(temp_even->next && temp_even->next->next)
        {
            temp_odd->next = temp_odd->next->next;
            temp_even->next = temp_even->next->next;
            temp_odd = temp_odd->next;
            temp_even = temp_even->next;
        }
        if(temp_even->next)
        {
            temp_odd->next = temp_odd->next->next;
            temp_odd = temp_odd->next;
        }
        temp_even->next = nullptr;
        temp_odd->next = even;        
        return head;
    }
};