> For the complete documentation index, see [llms.txt](https://mayanktyagi3111.gitbook.io/interview-prep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mayanktyagi3111.gitbook.io/interview-prep/strings-arrays-and-2-pointers/insert-delete-getrandom-o-1.md).

# Insert Delete GetRandom O(1)

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 (it's guaranteed that at least one element exists when this method is called). 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();
```

```java
public class RandomizedSet {
    ArrayList<Integer> nums;
    HashMap<Integer, Integer> map;
    Random random = new Random();

    // Initialize your data structure here.
    public RandomizedSet() {
        nums = new ArrayList<Integer>();
        map = new HashMap<Integer, Integer>();
    }

    // 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;
        map.put(val, nums.size());
        nums.add(val);
        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);
        if (index < nums.size() - 1) { // not the last one than swap the last one with this val
            int lastOne = nums.get(nums.size() - 1);
            nums.set(index, lastOne);
            map.put(lastOne, index);
        }
        map.remove(val);
        nums.remove(nums.size() - 1);
        return true;
    }

    // Get a random element from the set.
    public int getRandom() {
        // Chooses a random number between 0 and size-1
        return nums.get(random.nextInt(nums.size()));
    }
}
```