 # Insert Delete GetRandom O(1)

#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 .
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();
```

#2

The below code uses a hash-map and a slice. HashMap provides us the O(1) deletion/insertion time, while keeping the keys in a slice/array list provides us a way to randomized O(1). The key to this algorithm is we store the location of the value we are going to insert as a value to the key in the map.

``````import "math/rand"

type RandomizedSet struct {
rs map[int]int
keys []int
}

/** Initialize your data structure here. */
func Constructor() RandomizedSet {
return RandomizedSet{rs :  make(map[int]int), keys: make([]int, 0)}
}

/** Inserts a value to the set. Returns true if the set did not already contain the specified element. */
func (this *RandomizedSet) Insert(val int) bool {
if _, ok := this.rs[val]; !ok {
this.rs[val] = len(this.keys)
this.keys = append(this.keys, val)
return true
}
return false
}

/** Removes a value from the set. Returns true if the set contained the specified element. */
func (this *RandomizedSet) Remove(val int) bool {
if _, ok := this.rs[val];!ok {
return false
}
loc := this.rs[val]
if loc < len(this.keys)-1{
// set the last valuen to current loc, so we could delete last val
lastOne := this.keys[len(this.keys)-1]
this.keys[loc] = lastOne
this.rs[lastOne] = loc
}
this.keys = this.keys[0:len(this.keys)-1]
delete(this.rs, val)
return true
}

/** Get a random element from the set. */
func (this *RandomizedSet) GetRandom() int {
return this.keys[rand.Intn(len(this.rs))]
}

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

#3

Uses similar logic to use hash map and a list to store keys.

``````class RandomizedSet {

Map<Integer, Integer> map;
List<Integer> list;
java.util.Random rand = new java.util.Random();
/** Initialize your data structure here. */
public RandomizedSet() {
map = new HashMap<Integer, Integer>();
list = new ArrayList<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, list.size());
list.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 < list.size()-1){
int last = list.get(list.size()-1);
list.set(index, last);
map.put(last, index);
}
list.remove(list.size()-1);
map.remove(val);
return true;
}

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

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