Sort Characters By Frequency

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.