What is Hashmap?

A hashmap (also called a hash table) is a data structure that stores key → value pairs and supports fast insertion, deletion, and lookup on average.

In Python, the built-in implementation is dict.

mp = {"name": "Ali", "age": 20}
print(mp["name"])  # Ali

Introduction :

1. Key → Value mapping

   Each key points to exactly one value.

2. Fast average operations

   Insert / search / delete are typically O(1) average.

3. Keys are unique

   If you insert the same key again, the value gets overwritten.

4. Hashing

   A hash function converts a key into an index (bucket).

5. Collision handling

   If two keys map to the same bucket, the hashmap handles it using techniques like chaining or open addressing.

Core idea (Hashing)

A hash function takes a key and produces an integer index.

index = hash(key) % table_size

• Good hash functions spread keys evenly.
• Poor hashing increases collisions → slower performance.

Memory representation (Conceptual)

A hashmap uses an internal array of buckets.

• Each bucket can store:
  • a single (key, value) item, or
  • a list/chain of items (to handle collisions)