Day 136, 380, Insert Delete GetRandom O(1)

380. Insert Delete GetRandom O(1)

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

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

class RandomizedSet {
    private static Random random = new Random();
    private List<Integer> list;
    private Map<Integer, Integer> map;
    private int size;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        size = 0;
        list = new ArrayList<>();
        map = new HashMap<>();
    }
    
    /** 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, size);
        if (list.size() == size) {
            list.add(size, val);   
        }else {
            list.set(size, val);
        }
        size++;
        
        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 pos = map.get(val);
        int value = list.get(size - 1);
        list.set(pos, value);
        map.put(value, pos);
        size--;
        map.remove(val);
        
        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(random.nextInt(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();
 */

Follow up: duplicates are allowed.
Use Set as values in the map