<p>Given a string of English letters <code>s</code>, return <em>the <strong>greatest </strong>English letter which occurs as <strong>both</strong> a lowercase and uppercase letter in</em> <code>s</code>. The returned letter should be in <strong>uppercase</strong>. If no such letter exists, return <em>an empty string</em>.</p>

<p>An English letter <code>b</code> is <strong>greater</strong> than another letter <code>a</code> if <code>b</code> appears <strong>after</strong> <code>a</code> in the English alphabet.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;l<strong><u>Ee</u></strong>TcOd<u><strong>E</strong></u>&quot;
<strong>Output:</strong> &quot;E&quot;
<strong>Explanation:</strong>
The letter &#39;E&#39; is the only letter to appear in both lower and upper case.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;a<strong><u>rR</u></strong>AzFif&quot;
<strong>Output:</strong> &quot;R&quot;
<strong>Explanation:</strong>
The letter &#39;R&#39; is the greatest letter to appear in both lower and upper case.
Note that &#39;A&#39; and &#39;F&#39; also appear in both lower and upper case, but &#39;R&#39; is greater than &#39;F&#39; or &#39;A&#39;.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> s = &quot;AbCdEfGhIjK&quot;
<strong>Output:</strong> &quot;&quot;
<strong>Explanation:</strong>
There is no letter that appears in both lower and upper case.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= s.length &lt;= 1000</code></li>
	<li><code>s</code> consists of lowercase and uppercase English letters.</li>
</ul>