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# 380. 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. 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();
```

```cpp
class RandomizedSet {
public:
    /** Initialize your data structure here. */
    RandomizedSet() {
        srand(time(NULL));
    }
    
    /** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
    bool insert(int val) {
        if (m.count(val)) return false;
        nums.push_back(val);
        m[val] = nums.size() - 1;
        return true;
    }
    
    /** Removes a value from the set. Returns true if the set contained the specified element. */
    bool remove(int val) {
        if (!m.count(val)) return false;
        // swap the number-to-delete with the last number in the nums array
        // so that we can remove the last element in the array to save time
        int last = nums.back();
        if (last != val) {
            m[last] = m[val];
            nums[m[val]] = last;
        }
        nums.pop_back();
        m.erase(val);
        return true;
    }
    
    /** Get a random element from the set. */
    int getRandom() {
        return nums[rand() % nums.size()];
    }
private:
    vector<int> nums;
    unordered_map<int, int> m; // number -> index in nums array
};
```