Iterator for Combination LeetCode solution in Python

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class CombinationIterator:
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    def __init__(self, characters: str, combinationLength: int):
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        self.characters = characters
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        self.n = len(characters)
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        self.combinations = gen_combinations(self.n, combinationLength)
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        self.ind = len(self.combinations) - 1
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    def next(self) -> str:
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        s = ""
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        for i in range(self.n):
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            if self.combinations[self.ind][i] != "0":
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                s += self.characters[i]
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        self.ind -= 1
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        return s
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    def hasNext(self) -> bool:
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        return self.ind > -1
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def gen_combinations(l, n):
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    end = int("1" * l, 2)
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    ans = []
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    for i in range(end + 1):
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        b = bin(i)[2:]
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        if b.count("1") == n:
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            ans.append(b.zfill(l))
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    return ans