380. Insert Delete GetRandom O(1)

Design a data structure that supports all following operations in average O(1) time.

insert(val): Inserts an item val to the set if not already present.
remove(val): Removes an item val from the set if present.
getRandom: Returns a random element from current set of elements. Each element must have the same probability of being returned.
Example:

// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();

public class RandomizedSet {HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();ArrayList<Integer> al = new ArrayList<Integer>();Random rand = new Random();    /** Initialize your data structure here. */    public RandomizedSet() {    }    /** 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;        map.put(val, map.size());        al.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(!map.containsKey(val)) return false;        int loc = map.get(val);        if(loc != al.size()-1){            int last = al.get(al.size()-1);            al.set(loc, last);            map.put(last, loc);        }        map.remove(val);        al.remove(al.size()-1);        return true;    }    /** Get a random element from the set. */    public int getRandom() {        return al.get(rand.nextInt(al.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(); */