leetcode--Reorder List

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}.

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