Leetcode Insert Delete GetRandom O(1) problem solution

import random

class RandomizedSet(object):

    def __init__(self):
        """
        Initialize your data structure here.
        """
        self.nums,self.pos=[],{}

    def insert(self, val):
        """
        Inserts a value to the set. Returns true if the set did not already contain the specified element.
        :type val: int
        :rtype: bool
        """
        if val not in self.pos:
            self.pos[val]=len(self.nums)
            self.nums.append(val)
            return True
        return False

    def remove(self, val):
        """
        Removes a value from the set. Returns true if the set contained the specified element.
        :type val: int
        :rtype: bool
        """
        if val in self.pos:
            idx,last=self.pos[val],self.nums[-1]
            self.nums[idx],self.pos[last]=last,idx
            self.nums.pop()
            self.pos.pop(val,0)
            return True
        return False

    def getRandom(self):
        """
        Get a random element from the set.
        :rtype: int
        """
        return random.choice(self.nums)

class RandomizedSet {
    HashMap<Integer,Integer> hmap;
    ArrayList<Integer> arr;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        hmap=new HashMap<Integer,Integer>();
        arr=new ArrayList<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(hmap.get(val)!=null)
            return false;
         
         int s=arr.size();
         arr.add(val);
         hmap.put(val,s);
         return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        
        Integer idx=hmap.get(val);

        if(idx==null)
            return false;
        
        
        int size=arr.size();
        Integer last = arr.get(size-1);
        Collections.swap(arr,idx,size-1);
        
        hmap.put(last,idx);
        
        hmap.remove(val);
        arr.remove(size-1);


        return true;
    }
    
    /** Get a random element from the set. */
    public int getRandom() {
        if(arr.size()>0){
        Random rnd=new Random();
        int rnd_idx=rnd.nextInt(arr.size());
        return arr.get(rnd_idx);
        }
        return -1;
    }
}

class RandomizedSet {
private:
    unordered_set<int> elements;
    
public:
    
    RandomizedSet() {}
    
    bool insert(int val) {
        if(elements.find(val)!=elements.end()) return false;
        elements.insert(val);
        return true;
    }
    
    bool remove(int val) {
        if(elements.find(val)!=elements.end()) {
            elements.erase(val);
            return true;
        }
        return false;
    }
    
    int getRandom() {
        int randomNum = rand()%elements.size();
        auto it = elements.begin();
        advance(it,randomNum);
        return *it;
    }

};