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# 0380. Insert Delete GetRandom O(solutions/1)

<https://leetcode.com/problems/insert-delete-getrandom-o1>

## Description

Implement the `RandomizedSet` class:

* `RandomizedSet()` Initializes the `RandomizedSet` object.
* `bool insert(int val)` Inserts an item `val` into the set if not present. Returns `true` if the item was not present, `false` otherwise.
* `bool remove(int val)` Removes an item `val` from the set if present. Returns `true` if the item was present, `false` otherwise.
* `int getRandom()` Returns a random element from the current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the **same probability** of being returned.

You must implement the functions of the class such that each function works in **average** `O(1)` time complexity.

**Example 1:**

```
**Input**
["RandomizedSet", "insert", "remove", "insert", "getRandom", "remove", "insert", "getRandom"]
[[], [1], [2], [2], [], [1], [2], []]
**Output**
[null, true, false, true, 2, true, false, 2]
**Explanation**
RandomizedSet randomizedSet = new RandomizedSet();
randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomizedSet.remove(2); // Returns false as 2 does not exist in the set.
randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2].
randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly.
randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2].
randomizedSet.insert(2); // 2 was already in the set, so return false.
randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2.
```

**Constraints:**

* `-231 <= val <= 231 - 1`
* At most `2 *``105` calls will be made to `insert`, `remove`, and `getRandom`.
* There will be **at least one** element in the data structure when `getRandom` is called.

## ac

```java
class RandomizedSet {
    private Map<Integer, Integer> map;
    private List<Integer> list;
    private int len;
    private Random random;
    /** Initialize your data structure here. */
    public RandomizedSet() {
        map = new  HashMap<Integer, Integer>();
        list = new ArrayList<Integer>();
        len = 0;
        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) {
        if (map.containsKey(val)) return false;
        list.add(val);
        len++;
        map.put(val, len-1);
        return true;
    }

    /** Removes a value from the set. Returns true if the set contained the specified element. */
    public boolean remove(int val) {
        if (!map.containsKey(val)) return false;
        int index = map.get(val);
        int last = list.get(len-1);
        list.set(index, last);
        map.put(last, index);
        list.remove(len-1);
        len--;
        map.remove(val);
        return true;
    }

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

/**
 * 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();
 */
```