Data structure: insert, remove, contains, get random element, all at O(1)

Question

I was given this problem in an interview. How would you have answered?

Design a data structure that offers the following operations in O(1) time:

• inse

Answer 1

For this Question i will use two Data Structure

• HashMap
• ArrayList / Array / Double LinkedList.

Steps :-

1. Insertion :- Check if X is already present in HashMap --Time complexity O(1) . if not Present Then Add in end of ArrayList -- Time complexity O(1). add it in HashMap also x as key and last Index as a value -- Time complexity O(1).
2. Remove :- Check if X is present in HashMap --Time complexity O(1). If present then find the its index and remove it from HashMap --Time complexity O(1). swap this element with last element in ArrayList and remove the last element --Time complexity O(1). Update the index of last Element in HashMap --Time complexity O(1).
3. GetRandom :- Generate Random number from 0 to last index of ArrayList . return the ArrayList element at random index generated --Time complexity O(1).
4. Search :- See in HashMap for x as a key. --Time complexity O(1).

Code :-

import java.util.ArrayList;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Random;
import java.util.Scanner;


public class JavaApplication1 {

    public static void main(String args[]){
       Scanner sc = new Scanner(System.in);
        ArrayList al =new ArrayList();
        HashMap mp = new HashMap();  
        while(true){
            System.out.println("**menu**");
            System.out.println("1.insert");
            System.out.println("2.remove");
            System.out.println("3.search");
            System.out.println("4.rendom");
            int ch = sc.nextInt();
            switch(ch){
                case 1 : System.out.println("Enter the Element ");
                        int a = sc.nextInt();
                        if(mp.containsKey(a)){
                            System.out.println("Element is already present ");
                        }
                        else{
                            al.add(a);
                            mp.put(a, al.size()-1);

                        }
                        break;
                case 2 : System.out.println("Enter the Element Which u want to remove");
                        a = sc.nextInt();
                        if(mp.containsKey(a)){

                            int size = al.size();
                            int index = mp.get(a);

                            int last = al.get(size-1);
                            Collections.swap(al, index,  size-1);

                            al.remove(size-1);
                            mp.put(last, index);

                            System.out.println("Data Deleted");

                        }
                        else{
                            System.out.println("Data Not found");
                        }
                        break;
                case 3 : System.out.println("Enter the Element to Search");
                        a = sc.nextInt();
                        if(mp.containsKey(a)){
                            System.out.println(mp.get(a));
                        }
                        else{
                            System.out.println("Data Not Found");
                        }
                        break;
                case 4 : Random rm = new Random();
                        int index = rm.nextInt(al.size());
                        System.out.println(al.get(index));
                        break;

            }
        }
    }

}

-- Time complexity O(1). -- Space complexity O(N).