# Question

Formatted question description: https://leetcode.ca/all/380.html

Implement the RandomizedSet class:

• RandomizedSet() Initializes the RandomizedSet object.
• bool insert(int val) Inserts an item val into the set if not present. Returns true if the item was not present, false otherwise.
• bool remove(int val) Removes an item val from the set if present. Returns true if the item was present, false otherwise.
• int getRandom() Returns a random element from the 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.

You must implement the functions of the class such that each function works in average O(1) time complexity.

Example 1:

Input
["RandomizedSet", "insert", "remove", "insert", "getRandom", "remove", "insert", "getRandom"]
[[], [1], [2], [2], [], [1], [2], []]
Output
[null, true, false, true, 2, true, false, 2]

Explanation
RandomizedSet randomizedSet = new RandomizedSet();
randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully.
randomizedSet.remove(2); // Returns false as 2 does not exist in the set.
randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2].
randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly.
randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2].
randomizedSet.insert(2); // 2 was already in the set, so return false.
randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2.


Constraints:

• -231 <= val <= 231 - 1
• At most 2 * 105 calls will be made to insert, remove, and getRandom.
• There will be at least one element in the data structure when getRandom is called.

# Algorithm

Use a one-dimensional array and a HashMap, where

• the array is used to store the numbers,
• and the HashMap is used to establish the mapping between each number and its position in the array.

For insert() operation, first check whether this number already exists in HashMap,

• If it exists, return false directly,
• If it does not exist, insert it at the end of the array, and then create a mapping between the number and its position.

The delete() operation is tricky. It is necessary to determine whether it is in the HashMap first,

• If not, return false directly. Because the deletion of HashMap is constant time, but the array is not, in order to make the deletion of the array also constant level, in fact, the number to be deleted and the last number of the array are exchanged, and then the value in the corresponding HashMap is modified. You only need to delete the last element of the array, which ensures deletion within a constant time.

Returning a random() number is very simple for an array, as long as a position is randomly generated, the number at that position is returned.

# Code

• 
public class Insert_Delete_GetRandom_O_1 {

public static void main(String[] args) {

Insert_Delete_GetRandom_O_1 out = new Insert_Delete_GetRandom_O_1();

// Init an empty set.
RandomizedSet randomSet = out.new RandomizedSet();

System.out.println(randomSet.remove(0));
System.out.println(randomSet.remove(0));
System.out.println(randomSet.insert(0));
System.out.println(randomSet.insert(0));
System.out.println(randomSet.insert(0));

System.out.println(randomSet.getRandom());

System.out.println(randomSet.remove(0));
System.out.println(randomSet.insert(0));
}

class RandomizedSet {

List<Integer> list;
Map<Integer, Integer> map; // val => its index in list
Random random;

/** Initialize your data structure here. */
public RandomizedSet() {
list = new ArrayList<>();
map = new HashMap<>();
random = new Random();
}

/** 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;
}

int pos = list.size();

map.put(val, pos);

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;
}

// swap val to end of list, then remove it
int pos = map.get(val);
int lastVal = list.get(list.size() - 1);

list.set(pos, lastVal);
list.remove(list.size() - 1);

map.put(lastVal, pos); // @note: missed this line
map.remove(val);

return true;
}

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

/**
* 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();
*/
}

############

class RandomizedSet {
private Map<Integer, Integer> m = new HashMap<>();
private List<Integer> l = new ArrayList<>();
private Random rnd = new Random();

public RandomizedSet() {
}

public boolean insert(int val) {
if (m.containsKey(val)) {
return false;
}
m.put(val, l.size());
return true;
}

public boolean remove(int val) {
if (!m.containsKey(val)) {
return false;
}
int idx = m.get(val);
l.set(idx, l.get(l.size() - 1));
m.put(l.get(l.size() - 1), idx);
l.remove(l.size() - 1);
m.remove(val);
return true;
}

public int getRandom() {
int idx = rnd.nextInt(l.size());
return l.get(idx);
}
}

/**
* 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();
*/

• // OJ: https://leetcode.com/problems/insert-delete-getrandom-o1/
// Time: O(1) for all
// Space: O(N)
class RandomizedSet {
unordered_map<int, int> m; // value -> index
vector<int> v;
public:
RandomizedSet() {}

bool insert(int val) {
if (m.count(val)) return false;
m[val] = v.size();
v.push_back(val);
return true;
}

bool remove(int val) {
if (m.count(val) == 0) return false;
if (v.back() != val) {
m[v.back()] = m[val];
swap(v[m[val]], v.back());
}
v.pop_back();
m.erase(val);
return true;
}

int getRandom() {
return v[rand() % v.size()];
}
};

• from collections import defaultdict
from random import choice

class RandomizedSet(object):
def __init__(self):
"""
"""

# value (map) -> its index -> value (list)
self.d = {}
self.a = [] # introduce it purely for random

def insert(self, val):
"""
Inserts a value to the set. Returns true if the set did not already contain the specified element.
:type val: int
:rtype: bool
"""
if val in self.d:
return False
self.a.append(val)
self.d[val] = len(self.a) - 1
return True

def remove(self, val):
"""
Removes a value from the set. Returns true if the set contained the specified element.
:type val: int
:rtype: bool
"""
if val not in self.d:
return False
index = self.d[val]
self.a[index] = self.a[-1]
self.d[self.a[-1]] = index
self.a.pop() # delete in list
del self.d[val] # delete in dict
return True

def getRandom(self):
"""
Get a random element from the set.
:rtype: int
"""
return self.a[random.randrange(0, len(self.a))]
# return random.choice(self.a)

############

# Your RandomizedSet object will be instantiated and called as such:
# obj = RandomizedSet()
# param_1 = obj.insert(val)
# param_2 = obj.remove(val)
# param_3 = obj.getRandom()

'''
>>> import random
>>> random.choice([1,2,3,4,5])
3
>>> random.choice([1,2,3,4,5])
4
>>> random.choice([1,2,3,4,5])
4
'''
class RandomizedSet:
def __init__(self):
self.m = {}
self.l = []

def insert(self, val: int) -> bool:
if val in self.m:
return False
self.m[val] = len(self.l)
self.l.append(val)
return True

def remove(self, val: int) -> bool:
if val not in self.m:
return False
idx = self.m[val]
self.l[idx] = self.l[-1]
self.m[self.l[-1]] = idx
self.l.pop()
self.m.pop(val)
return True

def getRandom(self) -> int:
return random.choice(self.l)

# Your RandomizedSet object will be instantiated and called as such:
# obj = RandomizedSet()
# param_1 = obj.insert(val)
# param_2 = obj.remove(val)
# param_3 = obj.getRandom()

• type RandomizedSet struct {
m map[int]int
l []int
}

func Constructor() RandomizedSet {
return RandomizedSet{map[int]int{}, []int{} }
}

func (this *RandomizedSet) Insert(val int) bool {
if _, ok := this.m[val]; ok {
return false
}
this.m[val] = len(this.l)
this.l = append(this.l, val)
return true
}

func (this *RandomizedSet) Remove(val int) bool {
if _, ok := this.m[val]; !ok {
return false
}
idx := this.m[val]
this.l[idx] = this.l[len(this.l)-1]
this.m[this.l[len(this.l)-1]] = idx
this.l = this.l[:len(this.l)-1]
delete(this.m, val)
return true
}

func (this *RandomizedSet) GetRandom() int {
return this.l[rand.Intn(len(this.l))]
}

/**
* 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();
*/

• class RandomizedSet {
public map: Map<number, number>;
public arr: number[];
public index: number;

constructor() {
this.map = new Map();
this.arr = new Array(2 * 10 ** 5).fill(0);
this.index = -1;
}

insert(val: number): boolean {
const { map, arr } = this;
if (map.has(val)) {
return false;
}
this.index++;
arr[this.index] = val;
map.set(val, this.index);
return true;
}

remove(val: number): boolean {
const { arr, map, index } = this;
if (!map.has(val)) {
return false;
}
const i = map.get(val);
[arr[i], arr[index]] = [arr[index], arr[i]];
map.set(arr[i], i);
map.delete(arr[index]);
this.index--;
return true;
}

getRandom(): number {
const i = Math.floor(Math.random() * (this.index + 1));
return this.arr[i];
}
}

/**
* Your RandomizedSet object will be instantiated and called as such:
* var obj = new RandomizedSet()
* var param_1 = obj.insert(val)
* var param_2 = obj.remove(val)
* var param_3 = obj.getRandom()
*/


• use std::collections::HashSet;
use rand::Rng;

struct RandomizedSet {
list: HashSet<i32>
}

/**
* &self means the method takes an immutable reference.
* If you need a mutable reference, change it to &mut self instead.
*/
impl RandomizedSet {

fn new() -> Self {
Self {
list: HashSet::new()
}
}

fn insert(&mut self, val: i32) -> bool {
self.list.insert(val)
}

fn remove(&mut self, val: i32) -> bool {
self.list.remove(&val)
}

fn get_random(&self) -> i32 {
*self.list.iter().collect::<Vec<&i32>>()[i]
}
}

/**
* Your RandomizedSet object will be instantiated and called as such:
* let obj = RandomizedSet::new();
* let ret_1: bool = obj.insert(val);
* let ret_2: bool = obj.remove(val);
* let ret_3: i32 = obj.get_random();
*/

• public class RandomizedSet {
private Dictionary<int, int> d = new Dictionary<int, int>();
private List<int> q = new List<int>();

public RandomizedSet() {

}

public bool Insert(int val) {
if (d.ContainsKey(val)) {
return false;
}
return true;
}

public bool Remove(int val) {
if (!d.ContainsKey(val)) {
return false;
}
int i = d[val];
d[q[q.Count - 1]] = i;
q[i] = q[q.Count - 1];
q.RemoveAt(q.Count - 1);
d.Remove(val);
return true;
}

public int GetRandom() {
return q[new Random().Next(0, q.Count)];
}
}

/**
* 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();
*/