143. Reorder List

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}.

中间切一刀，前半部分记为L1，然后把后半部分反转记为L2，修改指向，轮流指向L1一个，指向L2一个

public class Solution {
    public void reorderList(ListNode head)
	{
		ListNode node=head;
		int cnt=0;
		while(node!=null)
		{
			cnt++;
			node=node.next;
		}
		ListNode rhead=null;
		node=head;
		for(int i=0;i<(cnt%2==0?cnt/2:cnt/2+1);i++)
			node=node.next;
		rhead=node;
		rhead=reverse(rhead);
		ListNode lhead=head;
		while(lhead!=null&&rhead!=null)
		{
			ListNode lnext=lhead.next;
			ListNode rnext=rhead.next;
			lhead.next=rhead;
			lhead=lnext;
			rhead.next=lhead;
			rhead=rnext;
		}
		if(lhead!=null)
			lhead.next=null;
	}
	
	private ListNode reverse(ListNode head)
	{
		if(head==null)
			return head;
		
		ListNode prev=head;
		ListNode p=head.next;
		ListNode pnext=null;
		while(p!=null)
		{
			pnext=p.next;
			p.next=prev;
			prev=p;
			p=pnext;
		}
		head.next=null;
		return prev;
	}
}