# Insert Delete GetRandom O(1)

## Introduction

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

insert(val): Inserts an item val to the set if not already present. remove(val): Removes an item val from the set if present. 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

Let’s solve it with two maps, one is to store association between val and pos, another is opposite one.

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

type RandomizedSet struct {
valToPos  map[int]int
posToVal  map[int]int
}

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

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

/** Removes a value from the set. Returns true if the set contained the specified element. */
func (t *RandomizedSet) Remove(val int) bool {
if pos, ok := t.valToPos[val]; ok {
delete(t.valToPos, val)
n := len(t.posToVal)
if n > 1 && n-1 != pos {
lastPos := n-1
lastVal := t.posToVal[lastPos]
t.posToVal[pos] = lastVal
t.valToPos[lastVal] = pos
delete(t.posToVal, lastPos)
} else {
delete(t.posToVal, pos)
}
return true
}
return false
}

/** Get a random element from the set. */
func (t *RandomizedSet) GetRandom() int {
n := len(t.posToVal)
switch n {
case 0:
return 0
case 1:
return t.posToVal[0]
default:
pos := rand.Intn(n)
return t.posToVal[pos]
}
}

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

Performance of this solution is:

```Runtime: 28 ms
Memory Usage: 7.9 MB```

Let’s improve performance by replacing the second map by array

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

type RandomizedSet struct {
valToPos  map[int]int
posToVal  []int
}

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

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

/** Removes a value from the set. Returns true if the set contained the specified element. */
func (t *RandomizedSet) Remove(val int) bool {
if pos, ok := t.valToPos[val]; ok {
delete(t.valToPos, val)
n := len(t.posToVal)
if n > 1 && n-1 != pos {
lastPos := n-1
lastVal := t.posToVal[lastPos]
t.posToVal[pos] = lastVal
t.valToPos[lastVal] = pos
}
t.posToVal = t.posToVal[:n-1]
return true
}
return false
}

/** Get a random element from the set. */
func (t *RandomizedSet) GetRandom() int {
n := len(t.posToVal)
switch n {
case 0:
return 0
case 1:
return t.posToVal[0]
default:
pos := rand.Intn(n)
return t.posToVal[pos]
}
}

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

Performance is the same:

```Runtime: 28 ms
Memory Usage: 7.6 MB```