380. Insert Delete GetRandom O(1)

import java.util.ArrayList;
import java.util.HashMap;
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 RandomizedSet {

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

    /** Initialize your data structure here. */
    public RandomizedSet() {
        nums = 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;
        }
        nums.add(val);
        map.put(val, nums.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 index = map.get(val);
        if (index != nums.size() - 1) {
            // Put the current last value at the index of the value to be removed.
            int lastVal = nums.get(nums.size() - 1);
            nums.set(index, lastVal);
            map.put(lastVal, index);
        }
        nums.remove(nums.size() - 1);
        map.remove(val);
        return true;
    }

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