/*
1.1 功能性需求: 支持插入, 删除, 返回随机值
1.2 非功能性需求: 3个操作都O(1)

2.1 插入删除O(1)-> HashMap
2.2 getRandom O(1) -> 维护数组域,
2.3 普通数组的查找是O(n),  删除是Π(n), 删除最后一个元素是O(1),
    因为本数组无需维护相互的关系性, 所以HashMap维护下对于val在数组中的位置, 删除时swap到最后去删除
*/

class RandomizedSet {

    Map<Integer, Integer> map ;
    List<Integer> list;
    Random random;

    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new HashMap<>();
        list = new ArrayList<>();
        random = new Random();
    }

    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    public boolean insert(int val) {
        Integer idx = map.get(val);
        if (idx != null) return false;
        list.add(val);
        map.put(val, list.size()-1);
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        Integer idx = map.get(val);
        if (idx == null) return false;
        map.put(list.get(list.size()-1), idx);
        Collections.swap(list, idx, list.size()-1);
        list.remove(list.size()-1);
        map.remove(val);
        return true;
    }

    /** Get a random element from the set. */
    public int getRandom() {
        return list.get(random.nextInt(list.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();
 */