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class Solution:
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    def frequencySort(self, s: str) -> str:
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        di = Counter(s)
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        # it wont strike immediately that this is a heap kind of question.
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        heap = []
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        heapq.heapify(heap)
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        for key, val in di.items():
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            heapq.heappush(heap, (-1 * val, key))
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        # n = len(s)
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        res = ""
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        # print(heap)
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        while len(heap):
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            val, ch = heapq.heappop(heap)
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            res += ch * (-1 * val)
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        return res