Evaluate the Bracket Pairs of a String LeetCode solution in Python

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class Solution:
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    def evaluate(self, s: str, knowledge: List[List[str]]) -> str:
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        knowledge = dict(knowledge)
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        answer, start = [], None
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        for i, char in enumerate(s):
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            if char == "(":
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                start = i + 1
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            elif char == ")":
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                answer.append(knowledge.get(s[start:i], "?"))
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                start = None
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            elif start is None:
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                answer.append(char)
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        return "".join(answer)