<?php
header("content-type:text/html;charset=utf-8");
/*
 *判斷一個列表是否是迴文結構 P48
 */
class Node{
    public $value;
    public $next;
    public function __construct($value)
    {
        $this->value = $value;
    }
}

function isPalindrome1($head){
    if($head == null){
        return false;
    }
    
 
$stack = new SplStack();
    $cur = $head;
    while ($cur != null){
        $stack->push($cur->value);
        $cur = $cur->next;
    }
    while ($head != null){
        if($head->value == $stack->pop()){
            $head = $head->next;
        }
        else{
            echo "單鏈表不是迴文結構";
            
 
return false;
        }
    }
    echo "單鏈表是迴文結構";
    return true;
}

function isPalindrome2($head){
    if($head == null || $head->next == null){
        echo "單鏈表是迴文結構";
        return true;
    }
    $right = $head->next;
    $cur = $head;
    while ($cur->next != null && $cur->next
 
->next != null){
        $right = $right ->next;
        $cur = $cur->next->next;
    }
    $stack = new SplStack();
    while ($right != null){
        $stack->push($right->value);
        $right = $right->next;
    }

    while (! $stack->isEmpty()){
        if($head->value != $stack->pop()){
            echo "單鏈表不是迴文結構";
            return false;
        }
        $head = $head->next;
    }
    echo "單鏈表是迴文結構";
    return true;

}

function isPalindrome3($head){
    if($head == null || $head->next == null){
        echo "單鏈表是迴文結構";
        return true;
    }
    $n1 = $head;
    $n2 = $head;
    while ($n2->next != null && $n2->next->next != null){
        $n1 = $n1->next;  //中間結點
        $n2 = $n2->next->next; //終點結點
    }
    $n3 = $n1->next;   //右邊連結串列的開頭
    $n1->next = null;
    //右邊連結串列進行翻轉
    $next = null;
    $pre = null;
    while ($n3 != null){
        $next = $n3->next;
        $n3->next = $pre;
        $pre = $n3;
        $n3 = $next;
    }
    while ($head != null && $pre != null){
        if($head->value != $pre->value){
            echo "單鏈表不是迴文結構";
            return false;
        }
        $head = $head->next;
        $pre = $pre->next;
    }
    echo "單鏈表是迴文結構";
    return true;


}
echo "單鏈表為：";
echo "</br>";
$head1 = new Node(1);
$head1->next = new Node(2);
$head1->next->next = new Node(3);
$head1->next->next->next = new Node(2);
$head1->next->next->next->next = new Node(1);
print_r($head1);
echo "</br>";
echo "</br>";
echo "方法1判斷是否迴文：";
echo "</br>";
echo isPalindrome1($head1);

$head2 = new Node(1);
$head2->next = new Node(2);
$head2->next->next = new Node(3);
$head2->next->next->next = new Node(2);
$head2->next->next->next->next = new Node(1);
echo "</br>";
echo "</br>";
echo "方法2判斷是否迴文：";
echo "</br>";
echo isPalindrome2($head2);

$head3 = new Node(1);
$head3->next = new Node(2);
$head3->next->next = new Node(3);
$head3->next->next->next = new Node(2);
$head3->next->next->next->next = new Node(1);
echo "</br>";
echo "</br>";
echo "方法3判斷是否迴文：";
echo "</br>";
echo isPalindrome3($head3);

輸出結果：