Mini Parser LeetCode solution in Python

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class Solution:
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    """
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    we can approach this problem using stack
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    """
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    def dfs(self, i, s):
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        res = NestedInteger()
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        while i < len(s):
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            if s[i] == "[":
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                y, i = self.dfs(i + 1, s)
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                res.add(y)
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            elif i < len(s) and s[i] == "]":
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                i += 1
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                return res, i
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            elif i < len(s) and s[i] == ",":
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                i += 1
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            else:
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                if i < len(s):
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                    start = i
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                    while s[i] != "," and s[i] != "]":
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                        i += 1
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                    res.add(NestedInteger(int(s[start:i])))
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        return res, i
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    def deserialize(self, s: str) -> NestedInteger:
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        if s[0] == "[":
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            res, i = self.dfs(1, s)
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            return res
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        else:
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            num = int(s)
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            res = NestedInteger()
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            res.setInteger(num)
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            return res