Longest Nice Substring LeetCode solution in Python

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class Solution:
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    def longestNiceSubstring(self, s: str) -> str:
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        def divcon(s):
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            # string with length 1 or less arent considered nice
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            if len(s) < 2:
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                return ""
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            pivot = []
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            # get every character that is not nice
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            for i, ch in enumerate(s):
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                if ch.isupper() and ch.lower() not in s:
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                    pivot.append(i)
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                if ch.islower() and ch.upper() not in s:
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                    pivot.append(i)
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            # if no such character return the string
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            if not pivot:
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                return s
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            # divide the string in half excluding the char that makes the string not nice
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            else:
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                mid = (len(pivot)) // 2
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                return max(
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                    divcon(s[: pivot[mid]]), divcon(s[pivot[mid] + 1 :]), key=len
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                )
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        return divcon(s)