# Insert Delete GetRandom O(1)

Hi Geeks! In this article, we going to see about how to insert, delete and get random elements with O(1) Time Complexity.

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();``````

### Solution

```public class RandomizedSet {

Random rand;
List<int> elements;
Dictionary<int,int> map;
/** Initialize your data structure here. */
public RandomizedSet() {
rand = new Random();
map = new Dictionary<int,int>();
elements = new List<int>();
}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
public bool Insert(int val) {
bool contain = map.ContainsKey(val);
if(!contain)
{
int index = elements.Count;
return true;
}
return false;
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
public bool Remove(int val) {
bool contain = map.ContainsKey(val);
if(!contain)
return false;
else
{
int index = map[val];
map.Remove(val);

//To remove at elements
if(index  == elements.Count-1)
elements.RemoveAt(index);
else
{
int lastElement = elements[elements.Count - 1];

elements[index] = lastElement;
map[lastElement] = index;

elements.RemoveAt(elements.Count-1);
}
return true;
}
}

/** Get a random element from the set. */
public int GetRandom() {
int randomIndex = rand.Next(0,elements.Count);
return elements[randomIndex];
}
}

/**
* Your RandomizedSet object will be instantiated and called as such:
* RandomizedSet obj = new RandomizedSet();
* bool param_1 = obj.Insert(val);
* bool param_2 = obj.Remove(val);
* int param_3 = obj.GetRandom();
*/
```

### Explanation

``````Elements : 1 2 3 4 5 6

Map

Key   Value
1       0
2       1
3       2
4       3
5       4
6       5

Remove(2)

int index = map[2]; //1
map.Remove(2);

Key   Value
1       0
3       2
4       3
5       4
6       5

int lastElement = elements[elements.Count-1]; //6
elements[index] = lastElement;

Elements : 1 6 3 4 5 6

elements.RemoveAt(elements.Count-1);
Elements : 1 6 3 4 5

map[lastElement] = index;

Key   Value
1       0
3       2
4       3
5       4
6       1

``````