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.

SOLUTION:

import heapq

class Solution:
    def frequencySort(self, s: str) -> str:
        n = len(s)
        freq = {}
        for c in s:
            freq[c] = freq.get(c, 0) + 1
        nlargest = heapq.nlargest(len(freq), [(value, key) for key, value in freq.items()])
        return "".join([c * k for k, c in nlargest])