调整列表顺序

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-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}.

public class Solution {
    public void reorderList(ListNode head) {
        if(head==null||head.next==null){
            return ;
        }
        ListNode fast=head.next;
        ListNode slow=head;
        while(fast!=null&&fast.next!=null){
            fast=fast.next.next;
            slow=slow.next;
        }
        fast=slow.next;
        slow.next=null;
        merge(head,reverse(fast));
    }
    public static ListNode merge(ListNode h1,ListNode h2){
        if(h1==null){
            return h2;
        }
        if(h2==null){
            return h1;
        }
        ListNode cur1=h1;
        ListNode cur2=h2;
        while(cur1!=null&&cur2!=null){
            ListNode next=cur2.next;
            cur2.next=cur1.next;
            cur1.next=cur2;
            cur1=cur1.next.next;
            cur2=next;
        }
        return cur1;
    }
    public static ListNode reverse(ListNode head){
        if(head==null||head.next==null){
            return head;
        }
        ListNode pre=null;
        ListNode next=null;
        while(head!=null){
            next=head.next;
            head.next=pre;
            pre=head;
            head=next;
        }
        return pre;
    }
}