451. Sort Characters By Frequency

https://leetcode.com/problems/sort-characters-by-frequency/description/

Description

Given a string, sort it in decreasing order based on the frequency of characters.

Example 1:

Input:
"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:
"cccaaa"
Output:
"cccaaa"
Explanation:
Both 'c' and 'a' appear three times, so "aaaccc" is also a valid answer.
Note that "cacaca" is incorrect, as the same characters must be together.

Example 3:

Input:
"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.

Solution

class Solution:
    def frequencySort(self, s):
        """
        :type s: str
        :rtype: str
        """
        cs_dict = dict()
        for c in s:
            cs_dict[c] = cs_dict.setdefault(c, 0) + 1
        cs_list = [(n, c) for c,n in cs_dict.items()]
        cs_list.sort(reverse=True)
        return ''.join(c*n for n,c in cs_list)