Leetcode #380. Insert Delete GetRandom O(1)

[yokostan]

Using HashSet, runtime beats 16%:

class RandomizedSet {
    HashSet<Integer> set;
    Random rand = new Random();

    /** Initialize your data structure here. */
    public RandomizedSet() {
        set = new HashSet<>();
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        if (set.contains(val)) return false;
        set.add(val);
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (set.contains(val)) {
            set.remove(val);
            return true;
        }
        return false;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        int size = set.size();
        int idx = rand.nextInt(size);
        Iterator<Integer> iter = set.iterator();
        for (int i = 0; i < idx; i++) {
            iter.next();
        }
        return iter.next();
    }
}

/**
 * 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();
 */

HashMap and ArrayList:

class RandomizedSet {
    HashMap<Integer, Integer> hash;
    ArrayList<Integer> list;
    int size;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        hash = new HashMap<>();
        list = new ArrayList<>();
        size = 0;
    }

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

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!hash.containsKey(val)) return false;
        int lastIndex = size - 1;
        int index = hash.get(val);
        if (index < lastIndex) {
            int lastVal = list.get(lastIndex);
            list.set(index, lastVal);
            hash.put(lastVal, index);
        }
        hash.remove(val);
        list.remove(lastIndex);
        size--;
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(new Random().nextInt(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();
 */