国外版

算法题（国外版）/longest-nice-substring.html

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+<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>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>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>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>