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class SmallestInfiniteSet {
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 public:
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  int cur;
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  set<int> s;
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  SmallestInfiniteSet() {
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    cur = 1;
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  }
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  int popSmallest() {
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    if (s.size()) {
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      int res = *s.begin();
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      s.erase(res);
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      return res;
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    } else {
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      cur += 1;
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      return cur - 1;
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    }
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  }
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  void addBack(int num) {
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    if (cur > num) s.insert(num);
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  }
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};