题解 | #判断一个链表是否为回文结构#

<?php

/*class ListNode{
    var $val;
    var $next = NULL;
    function __construct($x){
        $this->val = $x;
    }
}*/

/**
 * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
 *
 * 
 * @param head ListNode类 the head
 * @return bool布尔型
 */
function isPail( $head )
{
    // write code here
    $slow = $head;
    $fast = $head;
    while($fast && $fast->next){
        $fast = $fast->next->next;
        $slow = $slow->next;
    }
    $slow = reverseList($slow);
    $fast = $head;
    while($slow){
        if($fast->val==$slow->val){
            $fast = $fast->next;
            $slow = $slow->next;
        }else{
            return false;
        }
    }
    return true;
}
function reverseList($list){
    $pre = null;
    $cur = $list;
    while($cur){
        $next = $cur->next;
        $cur->next=$pre;
        $pre = $cur;
        $cur = $next;
    }
    return $pre;
}

快慢指针找到中间节点，后半段反转，然后比较前半段和后半段值是否相同，不相同返回false