Longest Nice Substring
<p>A string <code>s</code> is <strong>nice</strong> if, for every letter of the alphabet that <code>s</code> contains, it appears <strong>both</strong> in uppercase and lowercase. For example, <code>&quot;abABB&quot;</code> is nice because <code>&#39;A&#39;</code> and <code>&#39;a&#39;</code> appear, and <code>&#39;B&#39;</code> and <code>&#39;b&#39;</code> appear. However, <code>&quot;abA&quot;</code> is not because <code>&#39;b&#39;</code> appears, but <code>&#39;B&#39;</code> does not.</p>

<p>Given a string <code>s</code>, return <em>the longest <strong>substring</strong> of <code>s</code> that is <strong>nice</strong>. If there are multiple, return the substring of the <strong>earliest</strong> occurrence. If there are none, return an empty string</em>.</p>

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

<pre>
<strong>Input:</strong> s = &quot;YazaAay&quot;
<strong>Output:</strong> &quot;aAa&quot;
<strong>Explanation: </strong>&quot;aAa&quot; is a nice string because &#39;A/a&#39; is the only letter of the alphabet in s, and both &#39;A&#39; and &#39;a&#39; appear.
&quot;aAa&quot; is the longest nice substring.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;Bb&quot;
<strong>Output:</strong> &quot;Bb&quot;
<strong>Explanation:</strong> &quot;Bb&quot; is a nice string because both &#39;B&#39; and &#39;b&#39; appear. The whole string is a substring.
</pre>

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

<pre>
<strong>Input:</strong> s = &quot;c&quot;
<strong>Output:</strong> &quot;&quot;
<strong>Explanation:</strong> There are no nice substrings.
</pre>

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

<ul>
	<li><code>1 &lt;= s.length &lt;= 100</code></li>
	<li><code>s</code> consists of uppercase and lowercase English letters.</li>
</ul>
Bit Manipulation; Hash Table; String; Divide and Conquer; Sliding Window
function longestNiceSubstring(s: string): string {
    const n = s.length;
    let ans = '';
    for (let i = 0; i < n; i++) {
        let lower = 0,
            upper = 0;
        for (let j = i; j < n; j++) {
            const c = s.charCodeAt(j);
            if (c > 96) {
                lower |= 1 << (c - 97);
            } else {
                upper |= 1 << (c - 65);
            }
            if (lower == upper && j - i + 1 > ans.length) {
                ans = s.substring(i, j + 1);
            }
        }
    }
    return ans;
}
