Designing a HashMap without Built-in Libraries

Design a HashMap without using any built-in hash table libraries. To be specific, your design should include these functions:

• put(key, value): Insert a (key, value) pair into the HashMap. If the value already exists in the HashMap, update the value.
• get(key): Returns the value to which the specified key is mapped, or -1 if this map contains no mapping for the key.
• remove(key): Remove the mapping for the value key if this map contains the mapping for the key.

Examples:

Input: n = 8

• put(1, 1) 
• put(2, 2)
• get(1) 
• get(3)
• put(2, 1) 
• get(2)
• remove(2)
• get(2)

Output:
1
-1
1
-1

Explanation: MyHashMap hashMap = new MyHashMap();
hashMap.put(1, 1);          
hashMap.put(2, 2);         
hashMap.get(1); // returns 1
hashMap.get(3); // returns -1 (not found)
hashMap.put(2, 1); // update the existing value
hashMap.get(2); // returns 1 
hashMap.remove(2); // remove the mapping for 2
hashMap.get(2); // returns -1 (not found)

Input: n = 8

• put(1, 1) 
• put(2, 2)
• get(1) 
• get(2)
• put(3, 1) 
• get(3)
• remove(2)
• remove(3)

Output:
1
2
1

Approach: To solve the problem follow the below idea:

We will use array size upto 1e6. We will initialize all the values of the array by -1, as a value to denote no element currently at this position. Thus, we can use this array for all of the functions mentioned above.

Steps that were to follow the above approach:

• There can be at max 10^4 key-value pairs so we create an array of size 10^4+ 1 (0-based indexing).
• We initialize all the array values with -1 because by default they are 0 in an empty array and our value for a particular key can also be 0. So if we don’t initialize with something else it will give us the wrong outputs.
• After that, everything is a piece of cake. We put() the given value at the specified index key and similarly get() the value stored at the index key.
• When removing, we just initialize the specified index key back to -1.

Below is the code to implement the above steps:

C++

#include <iostream>
#include <vector>
using namespace std;

class MyHashMap {
    vector<int> mapArray;
public:
    MyHashMap() {
        mapArray = vector<int>(1000001, -1);
    }
    void put(int key, int value) {
        mapArray[key] = value;
    }
    int get(int key) {
        return mapArray[key];
    }
    void remove(int key) {
        mapArray[key] = -1;
    }
};

int main() {
    MyHashMap hashMap;
    hashMap.put(1, 1);
    hashMap.put(2, 2);
    cout << hashMap.get(1) << endl; // Output : 1
    cout << hashMap.get(3) << endl; // OUtput: -1 (not found)
    hashMap.put(2, 1);
    cout << hashMap.get(2) << endl; // Output: 1 (updated value)
    hashMap.remove(2);
    cout << hashMap.get(2) << endl; // Output: -1 (removed)
    return 0;
}

Java

// Java program to design HashMap
import java.io.*;
import java.util.*;

class MyHashMap {
    int[] mapArray;
    public MyHashMap()
    {
        mapArray = new int[1000001];
        Arrays.fill(mapArray, -1);
    }

    public void put(int key, int value)
    {
        mapArray[key] = value;
    }

    public int get(int key) { return mapArray[key]; }

    public void remove(int key) { mapArray[key] = -1; }

    // Drivers code
    public static void main(String args[])
    {
        MyHashMap hashMap = new MyHashMap();
        hashMap.put(1, 1);
        hashMap.put(2, 2);
        System.out.println(hashMap.get(1));
        System.out.println(hashMap.get(3));
        hashMap.put(2, 1);
        System.out.println(hashMap.get(2));
        hashMap.remove(2);
        System.out.println(hashMap.get(2));
    }
}

Python3

# python program to design HashMap

class MyHashMap:
    def __init__(self):
        self.mapArray = [-1] * 1000001

    def put(self, key, value):
        self.mapArray[key] = value

    def get(self, key):
        return self.mapArray[key]

    def remove(self, key):
        self.mapArray[key] = -1

# Driver code
if __name__ == "__main__":
    hashMap = MyHashMap()
    hashMap.put(1, 1)
    hashMap.put(2, 2)
    print(hashMap.get(1))  # Output: 1
    print(hashMap.get(3))  # Output: -1 (not found)
    hashMap.put(2, 1)
    print(hashMap.get(2))  # Output: 1 (updated value)
    hashMap.remove(2)
    print(hashMap.get(2))  # Output: -1 (removed)

C#

// C# program to design HashMap

using System;

public class MyHashMap {
    private int[] mapArray;

    public MyHashMap()
    {
        mapArray = new int[1000001];
        Array.Fill(mapArray, -1);
    }

    public void Add(int key, int value)
    {
        mapArray[key] = value;
    }

    public int Get(int key) { return mapArray[key]; }

    public void Remove(int key) { mapArray[key] = -1; }

    static public void Main()
    {

        // Code
        MyHashMap hashMap = new MyHashMap();
        hashMap.Add(1, 1);
        hashMap.Add(2, 2);
        Console.WriteLine(hashMap.Get(1));
        Console.WriteLine(hashMap.Get(3));
        hashMap.Add(2, 1);
        Console.WriteLine(hashMap.Get(2));
        hashMap.Remove(2);
        Console.WriteLine(hashMap.Get(2));
    }
}

Javascript

class MyHashMap {
    constructor() {
        this.mapArray = new Array(1000001).fill(-1);
    }
    put(key, value) {
        this.mapArray[key] = value;
    }
    get(key) {
        return this.mapArray[key];
    }
    remove(key) {
        this.mapArray[key] = -1;
    }
}

const hashMap = new MyHashMap();
hashMap.put(1, 1);
hashMap.put(2, 2);
console.log(hashMap.get(1)); // Output: 1
console.log(hashMap.get(3)); // Output: -1 (not found)
hashMap.put(2, 1);
console.log(hashMap.get(2)); // Output: 1 (updated value)
hashMap.remove(2);
console.log(hashMap.get(2)); // Output: -1 (removed)

Output

1
-1
1
-1

Time Complexity: O(1)
Auxiliary Space: O(1)