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;
    while(fast.next!=null&&fast.next.next!=null){
        slow=slow.next;
        fast=fast.next.next;
    }
     ListNode preMiddle=slow;   
    ListNode precurr=slow.next;
    while(precurr.next!=null){
        ListNode current=precurr.next;
        precurr.next=current.next;
        current.next=preMiddle.next;
        preMiddle.next=current;
    }
        slow=head;
        fast=preMiddle.next;
    while(slow!=preMiddle){
         preMiddle.next=fast.next;
                fast.next=slow.next;
                slow.next=fast;
                slow=fast.next;
                fast=preMiddle.next;
    }
    }
}