<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>

class Solution:
    def check(s: str) -> bool:
        lower = [False for _ in range(26)]
        upper = [False for _ in range(26)]
        for c in s:
            if c.islower():
                lower[ord(c) - ord('a')] = True
            else:
                upper[ord(c) - ord('A')] = True
        for i in range(26):
            if lower[i] != upper[i]:
                return False
        return True

    def longestNiceSubstring(self, s: str) -> str:
        res = ""
        for i in range(len(s)):
            for j in range(i+1, len(s)+1):
                if len(res) < j - i and Solution.check(s[i:j]):
                    res = s[i:j]
        return res

304