What is the difference between a hash set and a hash map?

A hash set and a hash map are both data structures that use hashing techniques to store and retrieve elements efficiently. However, they have some key differences in terms of their functionality and usage.

1. Purpose:
- Hash Set: A hash set is designed to store a collection of unique elements. It does not allow duplicate values and is primarily used to check for the presence of an element in the set.
- Hash Map: A hash map, on the other hand, is used to store key-value pairs. It allows duplicate values but ensures that each key is unique. It is commonly used to associate values with specific keys for efficient retrieval.

2. Structure:
- Hash Set: A hash set is implemented as a collection of unique elements, where each element is hashed and stored in a hash table. The elements are not associated with any specific value.
- Hash Map: A hash map consists of key-value pairs, where each key is hashed and used to determine the index in the hash table. The values associated with the keys are stored alongside the keys.

3. Operations:
- Hash Set: A hash set typically supports operations like adding an element, removing an element, and checking if an element exists in the set. It does not provide direct access to individual elements.
- Hash Map: A hash map supports operations like adding a key-value pair, removing a key-value pair, retrieving the value associated with a specific key, and checking if a key exists in the map.

4. Performance:
- Hash Set: The performance of a hash set is optimized for checking the presence of an element in the set. It provides constant-time complexity O(1) for operations like adding, removing, and checking if an element exists.
- Hash Map: The performance of a hash map is optimized for efficient key-value pair retrieval. It provides constant-time complexity O(1) for operations like adding, removing, and retrieving values based on keys.

In summary, the main difference between a hash set and a hash map lies in their purpose and structure. A hash set is used to store a collection of unique elements, while a hash map is used to store key-value pairs.