<?php

class ListNode {

public $val = 0;

public $next = null;

function __construct($val = 0, $next = null) {

$this->val = $val;

$this->next = $next;

}

}

/**

* @param ListNode $head

* @return ListNode

*/

function oddEvenList($head) {

if($head === null){

return null;

}

$cur = $head;

//当前位置计数

$i = 0;

//奇数位置链表

$oddhead = new ListNode();

$oddtail = $oddhead;

//偶数位置链表

$evenhead = new ListNode();

$eventail = $evenhead;

while ($cur !== null){

//当前位置计数

$i++;

//下一节点预报存

$tmp = $cur->next;

//判断位置奇数偶数

$is_odd = ($i % 2) ? true : false;

$cur->next = null;

//奇数位置放置于奇数列

if($is_odd){

$oddtail->next = $cur;

$oddtail = $cur;

}else{

$eventail->next = $cur;

$eventail = $cur;

}

//移动指针

$cur = $tmp;

}

$oddtail->next = $evenhead->next;

return $oddhead->next;

}