Increasing Decreasing String LeetCode solution in Python

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class Solution:
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    def sortString(self, s: str) -> str:
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        s = list(s)
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        result = ""
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        while s:
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            for letter in sorted(set(s)):
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                s.remove(letter)
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                result += letter
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            for letter in sorted(set(s), reverse=True):
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                s.remove(letter)
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                result += letter
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        return result