AC代碼：

class Solution(object):    def frequencySort(self, s):        """        :type s: str        :rtype: str        """        dict = {}        for j in range(len(s)):            if s[j] in dict:                dict[s[j]] += 1            else:                dict[s[j]] = 1        list = [[] for i in range(len(s))]        for key in dict:            list[dict[key]-1].append(key)        ans = ""        for j in range(len(list)-1,-1,-1):            if list[j] != []:                for k in list[j]:                    ans += (j+1)*k        return ans
思路：統計字符出現次數，然后建立一個索引數組，位置i存儲出現i+1次的字符，然后再逆序輸出該索引數組中的字符（i+1次）即可。