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