题目：

题解：

class Solution:
    def frequencySort(self, s: str) -> str:
        count = {}
        for c in s:
            count[c] = count.get(c, 0) + 1
        items = [(-val, key) for key, val in count.items()]
        heapq.heapify(items)
        res = ""
        while items:
            val, key = heapq.heappop()
            res += key * (-val)
        return res