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451. Sort Characters By Frequency

Difficulty Topics

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.

Solution

sort-characters-by-frequency.py
class Solution:
    def frequencySort(self, s: str) -> str:
        A = Counter(s)
        res = []

        for k, v in sorted(A.items(), key = lambda x:-x[1]):
            res += [k] * v

        return "".join(res)