QuickSet

.java

class QuickSet {
  int[] vi, iv;
  int count;

  public QuickSet(int N) {
    vi = new int[N]; // Value To Index
    iv = new int[N]; // index To Value
    count = 0;
  }

  void insert(int x) { // Runtime O(1)
    if (!lookup(x)) {
      vi[x] = count;
      iv[count] = x;
      count++;
    }
  }

  void remove(int x) { // Runtime O(1)
    if (lookup(x)) {
      int c = vi[x];

      count--;  // swap with the last one
      vi[iv[count]] = c;
      iv[c] = iv[count];
    }
  }

  boolean lookup(int x) { // Runtime O(1)
    return vi[x] < count && iv[vi[x]] == x;
  }

  void clear() { // Runtime O(1)
    count = 0;
  }

  void iterate(Consumer<Integer> f) { // Runtime O(count)
    for (int i = 0; i < count; i++) {
      f.accept(iv[i]);
    }
  }

  // test ----------------------------------------------------------------------
  public static void main(String[] args) {
    QuickSet s = new QuickSet(10);
    s.insert(5);
    s.insert(6);
    s.insert(7);
    s.remove(6);
    s.insert(9);
    s.iterate(v -> System.out.println(v));
    s.clear();
    s.insert(5);
    s.remove(5);
    s.insert(9);
    s.iterate(v -> System.out.println(v));
  }