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+<h2><a href="https://leetcode.com/problems/sort-characters-by-frequency/">451. Sort Characters By Frequency</a></h2><h3>Medium</h3><hr><div><p>Given a string <code>s</code>, sort it in <strong>decreasing order</strong> based on the <strong>frequency</strong> of the characters. The <strong>frequency</strong> of a character is the number of times it appears in the string.</p>
+
+<p>Return <em>the sorted string</em>. If there are multiple answers, return <em>any of them</em>.</p>
+
+<p>&nbsp;</p>
+<p><strong class="example">Example 1:</strong></p>
+
+<pre><strong>Input:</strong> s = "tree"
+<strong>Output:</strong> "eert"
+<strong>Explanation:</strong> 'e' appears twice while 'r' and 't' both appear once.
+So 'e' must appear before both 'r' and 't'. Therefore "eetr" is also a valid answer.
+</pre>
+
+<p><strong class="example">Example 2:</strong></p>
+
+<pre><strong>Input:</strong> s = "cccaaa"
+<strong>Output:</strong> "aaaccc"
+<strong>Explanation:</strong> Both 'c' and 'a' appear three times, so both "cccaaa" and "aaaccc" are valid answers.
+Note that "cacaca" is incorrect, as the same characters must be together.
+</pre>
+
+<p><strong class="example">Example 3:</strong></p>
+
+<pre><strong>Input:</strong> s = "Aabb"
+<strong>Output:</strong> "bbAa"
+<strong>Explanation:</strong> "bbaA" is also a valid answer, but "Aabb" is incorrect.
+Note that 'A' and 'a' are treated as two different characters.
+</pre>
+
+<p>&nbsp;</p>
+<p><strong>Constraints:</strong></p>
+
+<ul>
+	<li><code>1 &lt;= s.length &lt;= 5 * 10<sup>5</sup></code></li>
+	<li><code>s</code> consists of uppercase and lowercase English letters and digits.</li>
+</ul>
+</div>