# 0451. Sort Characters By Frequency

https://leetcode.com/problems/sort-characters-by-frequency

## Description

Given a string `s`, sort it in **decreasing order** based on the **frequency** of the characters. The **frequency** of a character is the number of times it appears in the string.

Return _the sorted string_. If there are multiple answers, return _any of them_.

**Example 1:**

```
**Input:** s = "tree"
**Output:** "eert"
**Explanation:** '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.
```

**Example 2:**

```
**Input:** s = "cccaaa"
**Output:** "aaaccc"
**Explanation:** 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.
```

**Example 3:**

```
**Input:** s = "Aabb"
**Output:** "bbAa"
**Explanation:** "bbaA" is also a valid answer, but "Aabb" is incorrect.
Note that 'A' and 'a' are treated as two different characters.
```

**Constraints:**

* `1 <= s.length <= 5 * 105`
* `s` consists of uppercase and lowercase English letters and digits.

## ac

```java
```
