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}.

原题链接：https://oj.leetcode.com/problems/reorder-list/

题目：给定一个单链表L: L0→L1→…→Ln-1→Ln,

将其重新排序成: L0→Ln→L1→Ln-1→L2→Ln-2→…

你不可以改变节点的值。

思路：把原链表从中间劈开，右半部分逆转，依次将逆转过的右半部分插入到左半部分中。

如 {1,2,3,4}——》{1,2}, {3,4} ——》{1,2} , {3,4}——》{1,3,2,4}

	public static void reorderList(ListNode head) {
		if (head == null || head.next == null)
			return;
		ListNode fast = head, slow = head;
		while (fast != null && fast.next != null) {
			slow = slow.next;
			fast = fast.next.next;
		}
		ListNode second = slow.next;
		slow.next = null;
		second = reverse(second);
		ListNode p1 = head;
		ListNode p2 = second;

		while (p2 != null) {
			ListNode temp1 = p1.next;
			ListNode temp2 = p2.next;

			p1.next = p2;
			p2.next = temp1;

			p1 = temp1;
			p2 = temp2;
		}
	}

	public static ListNode reverse(ListNode head) {
		if (head == null || head.next == null)
			return head;
		ListNode pre = head, current = head.next;
		while (current != null) {
			ListNode temp = current.next;
			current.next = pre;
			pre = current;
			current = temp;
		}
		head.next = null;
		return pre;
	}