Hash Tables

Hash table' is a data structure that allows for efficient insertion, deletion, and retrieval of items based on their keys. It uses a hash function to map each key to an index in an array, where the corresponding value is stored. Hash tables offer average-case constant-time complexity for operations such as insertions, deletions, and lookups.

Components of a Hash Table

Hash Function

A hash function takes a key as input and returns an integer, which corresponds to an index in the underlying array. A good hash function should distribute keys uniformly across the array, minimizing collisions. The choice of hash function depends on the types of keys being used.

Array

An array is the underlying data structure where the values are stored. The size of the array should be chosen carefully, as it affects the performance of the hash table. A larger array may reduce collisions but will consume more memory, while a smaller array may result in more collisions and slower performance.

Collision Resolution

Collisions occur when two or more keys are mapped to the same index in the array. To resolve collisions, several techniques can be employed:

• Separate chaining: Each array index contains a linked list or another data structure to store multiple values that map to the same index.
• Open addressing: When a collision occurs, the hash table searches for the next available slot in the array using a specified probing sequence (linear probing, quadratic probing, or double hashing).

Operations

Insert

To insert a key-value pair into the hash table, the hash function is applied to the key, and the value is stored at the corresponding index in the array. If a collision occurs, the chosen collision resolution technique is applied.

Lookup

To look up a value by its key, the hash function is applied to the key to find the corresponding index in the array. If a collision resolution technique is used, it may be necessary to search through the data structure at the index to find the correct value.

Delete

To delete a key-value pair, the hash function is applied to the key to find the corresponding index in the array. The value is then removed from the array or the data structure at the index. If a collision resolution technique is used, it may be necessary to search through the data structure at the index to find the correct value to delete.

Advantages and Disadvantages

Advantages

• Average-case constant-time complexity for insertions, deletions, and lookups.
• Efficient use of memory when the size of the array is chosen appropriately.

Disadvantages

• Collisions can cause a decrease in performance, especially if the hash function does not distribute keys uniformly or the array size is not chosen appropriately.
• Hash tables do not maintain any order of elements, unlike some other data structures, such as balanced search trees.

Example Hash Table Problems

• Two Sum problem
• Longest Substring Without Repeating Characters
• Group Anagrams
• Frequency Count of elements in an array

See Also

• Data Structures
• Algorithms