Given a singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-2→…

You must do this in-place without altering the nodes' values.

For example,
Given {1,2,3,4}, reorder it to {1,4,2,3}.

這道連結串列重排序問題可以拆分為以下三個小問題：

1. 使用快慢指標來找到連結串列的中點，並將連結串列從中點處斷開，形成兩個獨立的連結串列。

2. 將第二個鏈翻轉。

3. 將第二個連結串列的元素間隔地插入第一個連結串列中。

程式碼如下：

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    void reorderList(ListNode *head) {
        if (!head || !head->next || !head->next->next) return
 
;
        ListNode *fast = head;
        ListNode *slow = head;
        while (fast->next && fast->next->next) {
            slow = slow->next;
            fast = fast->next->next;
        }
        ListNode *mid = slow->next;
        slow->next = NULL;
        ListNode 
 
*last = mid;
        ListNode *pre = NULL;
        while (last) {
            ListNode *next = last->next;
            last->next = pre;
            pre = last;
            last = next;
        }
        while (head && pre) {
            ListNode *next = head->next;
            head->next = pre;
            pre = pre->next;
            head->next->next = next;
            head = next;
        }
    }
};