Design Bitset LeetCode solution in Java

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class Bitset {
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  int size;
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  Set<Integer> one = new HashSet<>();
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  Set<Integer> zero = new HashSet<>();
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  public Bitset(int size) {
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    this.size = size;
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    for (int i = 0; i < size; i++) zero.add(i);
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  }
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  public void fix(int idx) {
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    one.add(idx);
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    zero.remove(idx);
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  }
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  public void unfix(int idx) {
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    one.remove(idx);
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    zero.add(idx);
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  }
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  // swapping object's referrence is O(1)
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  public void flip() {
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    Set<Integer> s = one;
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    one = zero;
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    zero = s;
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  }
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  public boolean all() {
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    return one.size() == size;
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  }
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  public boolean one() {
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    return one.size() >= 1;
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  }
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  public int count() {
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    return one.size();
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  }
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  public String toString() {
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    StringBuilder sb = new StringBuilder();
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    for (int i = 0; i < size; i++) {
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      if (one.contains(i)) sb.append("1");
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      else if (zero.contains(i)) sb.append("0");
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    }
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    return sb.toString();
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  }
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}