381. Insert Delete GetRandom O(1) - Duplicates allowed

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Random;

/**
 * Using a ArrayList and HashMap
 *
 * Time Complexity: All function have average O(1)
 *
 * Space Complexity: O(N)
 *
 * N = Number of values currently stored in the data structure.
 */
class RandomizedCollection {

    ArrayList<Integer> nums;
    HashMap<Integer, HashSet<Integer>> map;
    Random rand;

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

    /**
     * Inserts a value to the collection. Returns true if the collection did not
     * already contain the specified element.
     */
    public boolean insert(int val) {
        boolean flag = false;
        if (!map.containsKey(val)) {
            flag = true;
            map.put(val, new HashSet<>());
        }
        nums.add(val);
        map.get(val).add(nums.size() - 1);
        return flag;
    }

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

        HashSet<Integer> indexes = map.get(val);
        int index = indexes.iterator().next();
        indexes.remove(index);
        if (indexes.size() == 0) {
            map.remove(val);
        }

        if (index != nums.size() - 1) {
            int lastVal = nums.get(nums.size() - 1);
            nums.set(index, lastVal);
            HashSet<Integer> lastValIndexes = map.get(lastVal);
            lastValIndexes.remove(nums.size() - 1);
            lastValIndexes.add(index);
        }
        nums.remove(nums.size() - 1);

        return true;
    }

    /** Get a random element from the collection. */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.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();
 */