Can you explain how the HashMap data structure works?

HashMap is a data structure that stores key-value pairs and provides quick access to values by key. Its operation is based on using a hash function that transforms the key into an array index (bucket).

When you add an element, the key is hashed, and the value is placed in the corresponding bucket. When searching by key, the hash function calculates the index, and the desired element is searched within the bucket.

To handle collisions (when different keys produce the same hash), chaining (linked list) or trees (in newer implementations) are usually used. This allows storing multiple elements in one bucket.

Example in Java:

HashMap<String, Integer> map = new HashMap<>();
map.put("apple", 3);
int count = map.get("apple"); // 3

HashMap provides an average operation complexity of about O(1) for insertion and search.