python 字典+sorted

while True:
    try:
        dic = {}
        for i in input():
            if i in dic:
                dic[i] += 1
            else:
                dic[i] = 1
        a = list(dic.keys())
        a.sort()
        sort = sorted(a, key = lambda x:dic[x], reverse=True)
        print(''.join(sort))
    except:
        break