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.