[LeetCode] 451. Sort Characters By Frequency

- Problem

451. 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 * 10^5
• s consists of uppercase and lowercase English letters and digits.

주어진 문자열 s를 문자 빈도 수가 많은 순서부터 차례대로 출력하는 문제이다.
여기서 대문자와 소문자는 다른 문자로 취급한다. 즉 A와 a는 다른 문자이다.
또한, lexicographical에 대해서는 신경쓰지 않아도 된다.

- 내 풀이

class Solution:
    def frequencySort(self, s: str) -> str:
        answer = ""
        count_string = collections.Counter(s)
        
        for key, value in sorted(count_string.items(), key = lambda x : -x[1]):
            answer += key * value
        
        return answer

• Counter 모듈을 이용하여 주어진 문자열 s의 문자 빈도 수를 계산한다.
• value 값을 내림차순으로 정렬한다.

- 결과