Add first-bad-version solution

Author: imajinyun

src/leetcode/FirstBadVersion.php

@@ -0,0 +1,34 @@
+<?php
+
+declare(strict_types=1);
+
+namespace leetcode;
+
+class FirstBadVersion
+{
+    public static function firstBadVersion(int $m, int $n): int
+    {
+        if ($m <= 0 || $n <= 0) {
+            return 0;
+        }
+        [$low, $high] = [1, $m];
+        while ($low <= $high) {
+            $mid = $low + (int)(($high - $low) / 2);
+            if (self::isBadVersion($mid, $n)) {
+                $high = $mid - 1;
+            } else {
+                $low = $mid + 1;
+            }
+        }
+
+        return $low;
+    }
+
+    public static function isBadVersion(int $m, int $n): bool
+    {
+        $map = array_fill(0, $n + 1, false);
+        $map[$n] = true;
+
+        return $map[$m];
+    }
+}

tests/leetcode/FirstBadVersionTest.php

@@ -0,0 +1,17 @@
+<?php
+
+declare(strict_types=1);
+
+namespace leetcode\tests;
+
+use PHPUnit\Framework\TestCase;
+use leetcode\FirstBadVersion;
+
+class FirstBadVersionTest extends TestCase
+{
+    public function testFirstBadVersion(): void
+    {
+        self::assertSame(4, FirstBadVersion::firstBadVersion(5, 4));
+        self::assertSame(1, FirstBadVersion::firstBadVersion(1, 1));
+    }
+}