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class SmallestInfiniteSet:
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    def __init__(self):
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        self.index = 1
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        self.heap = []
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    def popSmallest(self) -> int:
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        if self.heap:
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            return heapq.heappop(self.heap)
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        self.index += 1
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        return self.index - 1
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    def addBack(self, num: int) -> None:
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        if self.index > num and num not in self.heap:
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            heapq.heappush(self.heap, num)