给定一个字符串，请将字符串里的字符按照出现的频率降序排列。

class Solution:
    def frequencySort(self, s: str) -> str:
        d={}
        res=''
        for i in s:
            d[i]=d.get(i,0)+1
        d=sorted(d.items(),key=lambda k:-k[1])#字典排序
        for i in d:
            for j in range(i[1]):
                res+=i[0]
        return res