Reorder List leetcode java

题目：

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}

.

题解：

题目要重新按照 L0→Ln→L1→Ln-1→L2→Ln-2→…来排列，看例子1->2->3->4会变成1->4->2->3，拆开来看，是{1，2}和{4，3}的组合，而{4，3}是{3，4}的逆序。这样问题的解法就出来了。

第一步，将链表分为两部分。

第二步，将第二部分链表逆序。

第三步，将链表重新组合。

代码如下：

1 public void reorderList(ListNode head) {
2 if(head==null||head.next==null)
3 return;
4
5 ListNode slow=head, fast=head;
6 ListNode firsthalf = head;
7 while(fast.next!=null&&fast.next.next!=null){
8 slow = slow.next;
9 fast = fast.next.next;
}

ListNode secondhalf = slow.next;
slow.next = null;
secondhalf = reverseOrder(secondhalf);

while (secondhalf != null) {
ListNode temp1 = firsthalf.next;
ListNode temp2 = secondhalf.next;

firsthalf.next = secondhalf;
secondhalf.next = temp1;

firsthalf = temp1;
secondhalf = temp2;
}

}

public static ListNode reverseOrder(ListNode head) {

if (head == null || head.next == null)
return head;

ListNode pre = head;
ListNode curr = head.next;

while (curr != null) {
ListNode temp = curr.next;
curr.next = pre;
pre = curr;
curr = temp;
}

// set head node's next
head.next = null;

return pre;
} Reference：//http://www.programcreek.com/2013/12/in-place-reorder-a-singly-linked-list-in-java/