Insert Delete GetRandom O(1)

    private ArrayList<Integer> ls;
    private HashMap<Integer, Integer> map;
    private Random rand;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        ls = new ArrayList<>();
        map = new HashMap<>();
        rand = new Random();
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (map.containsKey(val)) return false;
        ls.add(val);
        map.put(val, ls.size() - 1);
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val)) return false;
        int idx = map.get(val);
        if (idx != ls.size() - 1) {
            int temp = ls.get(ls.size() - 1);
            ls.set(idx, temp);
            map.put(temp, idx);
        }
        map.remove(val);
        ls.remove(ls.size() - 1);
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return ls.get(rand.nextInt(ls.size()));
    }