# HashMap is a Data Structure every SWE should know.

### They provide an efficient way to store and retrieve data by mapping keys to values.

Here are 6 things you need to know about Hashmaps:

**1. Hashing**

Hashing is the process of converting a key into a hash code.

The hash code is an integer representing the key;

We use the key to compute the index in the hashmap's underlying array.

A good hashing function should distribute the keys uniformly to reduce collisions.

**2. Collisions**

Collisions occur when two different keys produce the same hash code.

Handling collisions is a critical aspect of hashmap implementation.

Resolution techniques:

- Chaining

- Open Addressing

**3. Time Complexity**

Hashmaps offer efficient:

- Lookup

- Insertion

- Deletion operations

On average, these operations have a time complexity of O(1).

But, in the worst case, when collisions are frequent, the time complexity can degrade to O(n), where n is the # of elements.

**4. Load Factor**

The load factor of a hashmap is the ratio of the number of elements stored to the array's size.

A high load factor increases the likelihood of collisions and degrades performance.

You can calculate the load factor using this formula.

n = number of elements stored

m = array's size

Resizing the hashmap and rehashing the elements can reduce this issue.

**5. Iterating through Elements**

Hashmaps do not maintain a specific order for their elements.

When iterating over a hashmap, it does not guarantee the order in retrieving elements.

If you need order, structures like linked lists or balanced trees can help.

**6. Hash Function Selection**

Choosing an appropriate hash function is important for a good distribution of keys.

The hash function should be fast and produce distributed hash codes to cut collisions.

**Common applications of using hashmaps:**

- Caching

- Indexing and Searching

- Databases

- Frequency Counting

- Graph Algorithms

- Distributed Systems

**In simple terms, they map keys to values.**

A hashmap is a collection of key-value pairs where each key is unique.

Have you used hashmaps?

Hashmaps are my favorite data structure.

Well explained Raul!