[leetcode] Reorder List

	public void reorderList(ListNode head) {
		if(head == null || head.next == null)
			return;
		ListNode fast, slow, pre = null;
		slow = fast = head;
		while(fast != null && fast.next != null)
		{
			pre = slow;
			slow = slow.next;
			fast = fast.next.next;
		}
		if(pre != null) pre.next = null;
		
		ListNode p = slow, pp = null; //Reverse
		while(p != null)
		{
			ListNode pn = p.next;
			p.next = pp;
			pp = p;
			p = pn;
		}
		
		p = head;
		ListNode p2 = pp;//Merge p, p2; p is longer or equal to p2
		
		while(p2 != null && p != null)
		{
			ListNode p2n = p2.next;
			ListNode pn = p.next;
			p.next = p2;
			if(pn != null) //p is exhausted
			{
				p2.next = pn;				
			}
			p = pn;
			p2 = p2n;
		}
	}