451. Sort Characters By Frequency

class Solution:
    def frequencySort(self, s):
        """
        :type s: str
        :rtype: str
        """
        a_dict = {}
        b_dict = {}
        new_string = ""
        new_list = []
        for i in range(len(s)):
            if s[i] not in a_dict:
                a_dict[s[i]] = 1
            else:
                a_dict[s[i]] += 1
        for (keys, values) in a_dict.items():
            if values not in b_dict:
                b_dict[values] = keys * values
            else:
                b_dict[values] = b_dict[values] + keys * values
        new_list = sorted(list(b_dict.keys()))
        for j in new_list:
            new_string = b_dict[j] + new_string
        return new_string