約数列挙

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vector<int> divisor(int n) {
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  vector<int> head, tail;
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  for (int i = 1; i * i <= n; i++) {
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    if (n % i == 0) {
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      head.push_back(i);
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      if (i * i != n)
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        tail.push_back(n / i);
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    }
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  }
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  head.insert(head.end(), tail.rbegin(), tail.rend());
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  return head;
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}