Hash Table

Hash Table Cheat Sheet

Hash tables, commonly referred to as hash maps, are a powerful data structure that pairs keys with values through a process known as hashing. By applying a hash function to a key, a hash table computes an index into an array of buckets or slots, enabling quick data retrieval. This efficiency in mapping keys to values makes hash tables an indispensable tool in software development.

Key Concepts

• Hash Function: A function that processes an input element to produce a hash code, determining the index where the value is stored in the hash table.

• Space-Time Tradeoff: Hash tables exemplify a tradeoff between space and time, offering rapid searches at the expense of additional memory to manage the hash table structure.

• Collision Resolution: Collisions occur when multiple keys hash to the same index. Techniques such as separate chaining (linking collided items using linked lists) or open addressing (finding the next available slot) are employed to resolve these collisions.

Implementations in Different Languages

• Golang: Maps.

• C++: Utilizes std::unordered_map for hash table implementation.

• Java: Employs java.util.Map, with java.util.HashMap being a common choice.

• Python: Implements hash tables using the dict type.

• JavaScript: Offers Object or Map for hash table functionalities.

Time Complexity

• Access: Direct access by hash code is not typically possible, making this operation not applicable.

• Search: O(1) on average, thanks to efficient hashing.

• Insert: O(1) on average, allowing for quick additions to the hash table.

• Remove: O(1) on average, facilitating fast removals.

Practice Problems

Leetcode	Difficulty
Two Sum	Easy
Valid Anagram	Easy
First Unique Character in a String	Easy
Intersection of Two Arrays II	Easy
Happy Number	Easy
Contains Duplicate	Easy
Group Anagrams	Medium
Top K Frequent Elements	Medium
Longest Substring Without Repeating Characters	Medium
Subarray Sum Equals K	Medium
Find All Anagrams in a String	Medium
Longest Palindrome	Medium
4Sum II	Medium
Partition Labels	Medium
Isomorphic Strings	Easy