451SortByFrequency.py

"""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."""

def frequencySort(self, s):
        """
        :type s: str
        :rtype: str
        """
        store = {}
        ans = ""
        for letter in s:
            if letter in store.keys():
                store[letter] += 1
            else:
                store[letter] = 1
        
        for key, value in [(k,store[k]) for k in sorted(store, key = store.get, reverse=True)]:
            for i in range (0, value):
                ans += key
        return ans