Palindrome

Palindrome.php

<?php

class ListElement
{
    /**
     * @var ListElement
     */
    private $next;

    private $value;

    /**
     * ListElement constructor.
     * @param $value
     */
    public function __construct($value, ListElement $next = null)
    {
        $this->value = $value;
        $this->next = $next;
    }

    /**
     * @return ListElement
     */
    public function getNext()
    {
        return $this->next;
    }

    /**
     * @return int
     */
    public function getValue()
    {
        return $this->value;
    }

    public function hasNext()
    {
        return $this->next instanceof ListElement;
    }
}

$head = new ListElement(3, new ListElement(4, new ListElement(2, new ListElement(4, new ListElement(3)))));

class Palindrome {

    private $left;

    public function isValid(ListElement $node){
        $this->left = $node;

        return $this->recurse($node);
    }

    function recurse(ListElement $node) {
        $previous = true;

        if ($node->hasNext()) {
            $previous = $this->recurse($node->getNext());
        }

        if ($this->left->getValue() === $node->getValue() && $previous === true) {
            $this->left = $this->left->getNext();
            return true;
        }

        return false;
    }
}

$palindrome = new Palindrome();

var_dump($palindrome->isValid($head));