Insert Delete GetRandom O(1) - Duplicates Allowed.java

class RandomizedCollection {

    /** Initialize your data structure here. */
    Map<Integer, Set<Integer>> map;
    List<Integer> list;

    public RandomizedCollection() {
        map = new HashMap<>();
        list = new ArrayList<>();
    }

    /** Inserts a value to the collection. Returns true if the collection did not already contain the specified element. */
    public boolean insert(int val) {
        map.computeIfAbsent(val, k -> new LinkedHashSet<>()).add(list.size());
        list.add(val);
        return map.get(val).size() == 1;
    }

    /** Removes a value from the collection. Returns true if the collection contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val) || map.get(val).size() == 0) {
            return false;
        }

        int idxToRemove = map.get(val).iterator().next();
        map.get(val).remove(idxToRemove);
        int lastVal = list.get(list.size() - 1);
        list.set(idxToRemove, lastVal);
        map.get(lastVal).add(idxToRemove);
        map.get(lastVal).remove(list.size() - 1);
        list.remove(list.size() - 1);
        
        return true;
    }

    /** Get a random element from the collection. */
    public int getRandom() {
        return list.get(new java.util.Random().nextInt(list.size()));
    }
}

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