字典儲存 字典排序 遍歷排序後的list

class Solution:
    def frequencySort(self, s):
        """
        :type s: str
        :rtype: str
        """
        dict1={}
        for c in s:
            if c in dict1:
                dict1[c]+=1
            else:
                dict1[c]=1
        b=sorted(dict1.items(),key=lambda x:x[1],reverse=True)
        res=''
        for item in b:
            res+=item[0]*item[1]
        return res