Add palindrome-linked-list solution

Author: imajinyun

Diff for: src/leetcode/PalindromeLinkedList.php

@@ -0,0 +1,72 @@
+<?php
+
+declare(strict_types=1);
+
+namespace leetcode;
+
+use leetcode\util\ListNode;
+
+class PalindromeLinkedList
+{
+    public static function isPalindrome(ListNode $head): bool
+    {
+        $fast = $head;
+        $slow = $head;
+        while ($fast && $fast->next) {
+            $fast = $fast->next->next;
+            $slow = $slow->next;
+        }
+        if ($fast) {
+            $slow = $slow->next;
+        }
+        $fast = $head;
+        $slow = self::helper($slow);
+        while ($slow) {
+            if ($fast->val !== $slow->val) {
+                return false;
+            }
+            $fast = $fast->next;
+            $slow = $slow->next;
+        }
+
+        return true;
+    }
+
+    /**
+     * Note: Time Limit Exceeded.
+     *
+     * @param \leetcode\util\ListNode $head
+     *
+     * @return bool
+     */
+    public static function isPalindrome2(ListNode $head): bool
+    {
+        $node = $head;
+        $queue = [];
+        while ($node) {
+            array_push($queue, $node);
+            $node = $node->next;
+        }
+        while (count($queue) >= 2) {
+            [$p, $q] = [array_shift($queue), array_pop($queue)];
+            if ($p && $q && $p->val != $q->val) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    private static function helper(ListNode $head): ?ListNode
+    {
+        $prev = null;
+        while ($head) {
+            $next = $head->next;
+            $head->next = $prev;
+            $prev = $head;
+            $head = $next;
+        }
+
+        return $prev;
+    }
+}

Diff for: tests/leetcode/PalindromeLinkedListTest.php

@@ -0,0 +1,30 @@
+<?php
+
+declare(strict_types=1);
+
+namespace leetcode\tests;
+
+use PHPUnit\Framework\TestCase;
+use leetcode\PalindromeLinkedList;
+use leetcode\util\ListNode;
+
+class PalindromeLinkedListTest extends TestCase
+{
+    public function testIsPalindrome(): void
+    {
+        $head1 = new ListNode(1, new ListNode(2, new ListNode(2, new ListNode(1))));
+        self::assertTrue(PalindromeLinkedList::isPalindrome($head1));
+
+        $head2 = new ListNode(1, new ListNode(2));
+        self::assertFalse(PalindromeLinkedList::isPalindrome($head2));
+    }
+
+    public function testIsPalindrome2(): void
+    {
+        $head1 = new ListNode(1, new ListNode(2, new ListNode(2, new ListNode(1))));
+        self::assertTrue(PalindromeLinkedList::isPalindrome2($head1));
+
+        $head2 = new ListNode(1, new ListNode(2));
+        self::assertFalse(PalindromeLinkedList::isPalindrome2($head2));
+    }
+}