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}.

实现最终的链表需要三步：将链表分成前后两段，反转后半部分，合并两个链表。

public void reorderList(ListNode head) {
	if(head==null&&head.next!=null)
		return;
	ListNode fast=head,slow=head;
	while(fast!=null&&fast.next!=null&&fast.next.next!=null){
		fast=fast.next.next;
		slow=slow.next;
	}
	ListNode t1=slow.next;
	slow.next=null;
	ListNode t2=reverseList(t1);
	ListNode t=head;
	while(t2!=null){
		ListNode t3=t.next,t4=t2.next;
		t.next=t2;
		t2.next=t3;
		t=t3;
		t2=t4;
	}
}
public ListNode reverseList(ListNode head){
	ListNode newHead=null;
	while(head!=null){
		ListNode t=head.next;
		head.next=newHead;
		newHead=head;
		head=t;
	}
	return newHead;
}