657. insert delete getrandom o(1)

public class  {
    
    insert, delete: hashset
    getRandom: need index-- arraylist
    index & hash: hasemap:<val, index>
    */
    
    pick random number
    Random rand = new Random():
    rand.nextInt(40); 0-39
    */
    private Map<Integer, Integer> map;
    private List<Integer> list;
    private Random rand;

    public () {
        // do intialization if necessary
        map = new HashMap<Integer, Integer>();
        list = new ArrayList<Integer>();
        rand = new Random();

    }

    
     * @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) {
        // write your code here
        if (map.containsKey(val)) {
            return false;
        }
        map.put(val, list.size());
        list.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) {
        // write your code here
        if (!map.containsKey(val)) {
            return false;
        }
        int index = map.get(val);
        //!!!!!!!!!!!!!!!!!!!!!!!
        // 如果remove掉的不是在list里最后的那个，其他的index会受到影响
        // 把这个element和最后那个交换
        // map(last, index), list(index, last)
        if (index < list.size() - 1) {
            int last = list.get(list.size() - 1);
            map.put(last, index);
            list.set(index, last);
        }
        map.remove(val);
        list.remove(list.size() - 1);
        return true;
    }

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

/**
 * Your RandomizedSet object will be instantiated and called as such:
 * RandomizedSet obj = new RandomizedSet();
 * boolean param = obj.insert(val);
 * boolean param = obj.remove(val);
 * int param = obj.getRandom();
 */