Add reverse-string-ii solution

Author: imajinyun

src/leetcode/ReverseStringII.php

@@ -0,0 +1,26 @@
+<?php
+
+declare(strict_types=1);
+
+namespace leetcode;
+
+class ReverseStringII
+{
+    public static function reverseStr(string $s, int $k): string
+    {
+        if (empty($s) || $k <= 0) {
+            return '';
+        }
+        $n = strlen($s);
+        [$m, $n] = [0, strlen($s)];
+        for (; $m < $n; $m += 2 * $k) {
+            for ($i = $m, $j = min($m + $k - 1, $n - 1); $i < $j; $i++, $j--) {
+                $t = $s[$i];
+                $s[$i] = $s[$j];
+                $s[$j] = $t;
+            }
+        }
+
+        return $s;
+    }
+}

tests/leetcode/ReverseStringIITest.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace leetcode\tests;
+
+use leetcode\ReverseStringII;
+use PHPUnit\Framework\TestCase;
+
+class ReverseStringIITest extends TestCase
+{
+    public function testReverseStr(): void
+    {
+        self::assertSame('bacdfeg', ReverseStringII::reverseStr('abcdefg', 2));
+        self::assertSame('bacd', ReverseStringII::reverseStr('abcd', 2));
+    }
+}