leetcode380. 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 InsertDeleteGetRandom_380 {
    private List<Integer> list;
    private Map<Integer, Integer> hash;
    
    public InsertDeleteGetRandom_380() {
        list = new ArrayList<Integer>();
        hash = new HashMap<Integer, Integer>();
    }
    
     /** 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;
        }
        list.add(val);
        hash.put(val, list.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(!hash.containsKey(val)){
            return false;
        }
        int position = hash.get(val);
        if(position != list.size()-1) {
            int last = list.get(list.size()-1);
            list.set(position, last);
            hash.put(last, position);
        }
        list.remove(list.size()-1);
        hash.remove(val);

        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        int position = (int)Math.floor((Math.random() * list.size()));
        return list.get(position);
    }
}