from collections import Counter
c = Counter(input())
result = sorted(c.items(), key=lambda x: (-x[1], x[0]))
print("".join(x[0] for x in result))

例如result = [('c', 3), ('d', 3), ('a', 2)]

用-x[1]就可以改成按数字降序， x[0]按字母升序， sorted 条件按顺序来的