Solution.java

class RandomizedSet {
    private Map<Integer, Integer> d = new HashMap<>();
    private List<Integer> q = new ArrayList<>();
    private Random rnd = new Random();

    public RandomizedSet() {
    }

    public boolean insert(int val) {
        if (d.containsKey(val)) {
            return false;
        }
        d.put(val, q.size());
        q.add(val);
        return true;
    }

    public boolean remove(int val) {
        if (!d.containsKey(val)) {
            return false;
        }
        int i = d.get(val);
        d.put(q.get(q.size() - 1), i);
        q.set(i, q.get(q.size() - 1));
        q.remove(q.size() - 1);
        d.remove(val);
        return true;
    }

    public int getRandom() {
        return q.get(rnd.nextInt(q.size()));
    }
}

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param_1 = obj.insert(val);
 * boolean param_2 = obj.remove(val);
 * int param_3 = obj.getRandom();
 */