Insert Delete GetRandom O(1)

Java

public class RandomizedSet {
    ArrayList<Integer> nums;
    HashMap<Integer, Integer> nums2Int;
    Random rand;
    public RandomizedSet() {
        nums = new ArrayList<>();
        nums2Int = new HashMap<>();
        rand = new Random();// do intialization if necessary
    }

    /*
     * @param val: a value to the set
     * @return: true if the set did not already contain the specified element or false
     */
    public boolean insert(int val) {
        if(nums.contains(val)){
            return false;
        }
        
        nums2Int.put(val,nums.size());
        nums.add(val);
        return true;
        
    }

    /*
     * @param val: a value from the set
     * @return: true if the set contained the specified element or false
     */
    public boolean remove(int val) {
        if(nums2Int.containsKey(val) == false){
            return false;
        }
        int index = nums2Int.get(val);
        if(index < nums.size() - 1){
            int last = nums.get(nums.size()-1);
            nums2Int.put(last, index);
            nums.set(index , last);
            
        }
        nums2Int.remove(nums.size() - 1);
        nums.remove(nums.size() - 1);
        return true;
        
    }

    /*
     * @return: Get a random element from the set
     */
    public int getRandom() {
        return nums.get(rand.nextInt(nums.size()));// write your code here
    }
}