LeetCode - 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();

用一个 hashmap和一个arraylist, hashmap用来保存元素和其在list中的index， list保存元素。注意remove function中如何用O(1)的方法从list中remove元素。

class RandomizedSet {

    Map<Integer, Integer> map;
    List<Integer> list;
    int size;
    
    
    /** Initialize your data structure here. */
    public RandomizedSet() {
        this.map = new HashMap<>();
        this.list = new ArrayList<>();
        int size = 0;
    }
    
    /** 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)){
            list.add(val);
            size++;
            map.put(val, size-1);
            return true;
        }
        return false;
        
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if(map.containsKey(val)){
            int index = map.remove(val);
            if(index != size - 1){
                map.put(list.get(size-1), index);
                list.set(index, list.get(size-1));
                list.remove(size - 1);
                
            }
            else{
                list.remove(size - 1);
            }
            size--;
            return true;
        }
        return false;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        Random rdm = new Random();
        return list.get(rdm.nextInt(map.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();
 */