Python 101: hash table

A hash table is primarily built using a dynamic array as its core structure, but it may use linked lists or trees (like balanced trees) for collision resolution. Let’s break this down in detail:

1. Core Structure: Dynamic Array

• What: A hash table stores its key-value pairs in an internal array, where the keys are mapped to specific indices in the array using a hash function.
• Why: The dynamic array allows for constant-time access by index (O(1)) on average, making it ideal for quickly locating a key’s position in the table.
• Dynamic Resizing: As the number of key-value pairs grows, the internal array can resize dynamically to maintain efficiency (this is why it’s called a dynamic array).

2. Collision Handling: Linked List or Balanced Tree

Hash tables must handle collisions, which occur when two or more keys map to the same index in the array. Different methods are used to resolve these collisions:

• Chaining with Linked Lists:

  • What: The most common technique is chaining, where each bucket in the array points to a linked list (or a similar structure like a dynamic array) that stores all key-value pairs that hash to the same index.
  • How: If multiple keys hash to the same index, they are added to the linked list for that bucket.

  Example:

  Index 0: [None]
  Index 1: [(key1, value1), (key2, value2)]  --> Linked List for collisions
  Index 2: [(key3, value3)]

• Chaining with Trees (Advanced):

  • What: Some implementations, like Java’s HashMap, may switch to a balanced tree (e.g., red-black tree) for collision handling when there are too many collisions at a single index. This ensures that even in the worst case, lookup time stays efficient (O(log n)).
  • Why: Using a tree structure for heavily loaded buckets helps prevent performance degradation when many collisions occur.

3. Open Addressing (No Linked List or Tree):

Another method of handling collisions is open addressing, where no additional structure (like a linked list or tree) is used. Instead, the hash table probes for the next available slot in the array when a collision occurs. This method avoids the need for a linked list or tree but can degrade performance as the table fills up.

Summary:

• Dynamic Array: The hash table’s main structure is a dynamic array, which stores key-value pairs at hashed indices.
• Linked List or Tree: These are used to handle collisions (linked lists are more common, but trees are used in advanced cases).
• Open Addressing: Another method for handling collisions without using linked lists or trees.

Comparison of Components: