【一天一道】You must do this in-place without altering the nodes' values

 

Given a singly linked list L: L 0→L 1→…→L n-1→L n,
reorder it to: L 0→L n →L 1→L n-1→L 2→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}.

题目比较简单，但是要求时间在1s内，空间在32768k内，完全不达标。。。

step1：快慢指针找中点；

step2：链表翻转；

step3：链表合并；

 

public class Solution {
    public void reorderList(ListNode head) {
        ListNode slow, quik, cur, pre;
        ListNode curHead;
        slow = head;
        quik = head;
        if(head ==null || head.next == null){
            return;
        }
        while(quik.next != null && quik.next.next != null){
            quik = quik.next.next;
            slow = slow.next;
        }
        cur = slow.next;
        slow.next = null;
        pre = null;
        while(cur != null){
            ListNode curPost = cur.next;
            cur.next = pre;
            pre = cur;
            cur = curPost;
        }
        curHead = head;
        while(curHead != null && pre != null){
            ListNode preNext = pre.next;
            pre.next = curHead.next;
            curHead.next = pre;
            curHead = pre.next;
            pre = preNext;
        }   
    }
}