Core Data Structures

Deeply understand arrays, linked lists, stacks, queues, hash tables, and sets — not just how to use them (you already do), but how they work internally, their memory layout, and their performance characteristics. Implement each from scratch in Ruby. Ruby's Hash is arguably the most important data structure in Rails. Every params hash, every session, every ActiveRecord attributes hash — they all depend on O(1) average-case lookup. Understanding how hash collisions degrade performance to O(n) explains why choosing good hash keys matters. Understanding how Set is implemented as a Hash with nil values explains why user_ids.to_set.include?(id) is O(1) while user_ids.include?(id) is O(n). These are not academic exercises — they are production performance decisions you make every day.