# 380. Insert Delete GetRandom O(1) (Medium)

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

Design a data structure that supports all following operations in average O(1) time.

1. `insert(val)`: Inserts an item val to the set if not already present.
2. `remove(val)`: Removes an item val from the set if present.
3. `getRandom`: Returns a random element from current set of elements. Each element must have the same probability of being returned.

Example:

```// Init an empty set.
RandomizedSet randomSet = new RandomizedSet();

// Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomSet.insert(1);

// Returns false as 2 does not exist in the set.
randomSet.remove(2);

// Inserts 2 to the set, returns true. Set now contains [1,2].
randomSet.insert(2);

// getRandom should return either 1 or 2 randomly.
randomSet.getRandom();

// Removes 1 from the set, returns true. Set now contains [2].
randomSet.remove(1);

// 2 was already in the set, so return false.
randomSet.insert(2);

// Since 2 is the only number in the set, getRandom always return 2.
randomSet.getRandom();
```

## Solutions

``````class RandomizedSet {

List<Integer> values = new ArrayList<>();
Set<Integer> presentValues = new HashSet<>();

/** Initialize your data structure here. */
public RandomizedSet() {

}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
public boolean insert(int val) {
return false;
}

return true;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
public boolean remove(int val) {
if (!presentValues.remove(val)) {
return false;
}

// it must be converted to object type. primitive int represents the index, not the element.
values.remove(Integer.valueOf(val));

return true;
}

/** Get a random element from the set. */
public int getRandom() {
int len = presentValues.size();

// index should not be len + 1 because it already overstep the boundary of the list.
int ridx = new Random().nextInt(len);

return values.get(ridx);
}
}
``````