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();

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