leetCode--reorder-list

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}.

/**
 * Definition for singly-linked list.
 * class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public void reorderList(ListNode head) {
        if(head==null||head.next==null){
            return;
        }
        ListNode slow=head;
        ListNode fast=head.next;
        //找到中点
        while(fast!=null&&fast.next!=null){
            slow=slow.next;
            fast=fast.next.next;
        }
        ListNode after=slow.next;
        slow.next=null;
        ListNode pre=null;
        //后半部分转置
        while(after!=null){
            ListNode tmp=after.next;
            after.next=pre;
            pre=after;
            after=tmp;
        }
        //合并
        ListNode first=head;
        after=pre;
        while(first!=null&&after!=null){
            ListNode firstTmp=first.next;
            ListNode afterTmp=after.next;
            first.next=after;
            first=firstTmp;
            after.next=first;
            after=afterTmp;
        }
    }
}