這個題是基本技能的一個集合，用到了快慢指針找中點，反轉鏈表和合並鏈表

一些鏈表的拼接過程要熟練

public void reorderList(ListNode head) {
        /*
        先找到中點，然後反轉後部分，然後組合鏈表
         */
        if (head==null) return;
        //第一步，快慢指針找中點
        ListNode slow = head;
        ListNode fast = head;
        while (fast!=null&&fast.next!=null
 
)
        {
            slow = slow.next;
            fast = fast.next.next;
        }
        ListNode back = slow.next;
        slow.next = null;
        //反轉後邊
        ListNode pre = null;
        while (back!=null)
        {
            ListNode next = back.next;
            back.next = pre;
            pre 
 
= back;
            back = next;
        }
        ListNode res = head;
        //拼接
        while (res!=null&&pre!=null)
        {
                ListNode t1 = res.next;
                ListNode t2 = pre.next;
                res.next = pre;
                pre.next = t1;
                pre 
 
= t2;
                res = t1;
        }
    }

[LeetCode]143. Reorder List