A hash table computes where a key must be rather than searching for it — until two keys land in the same slot and everything becomes a question of what to do about it. Interactive walkthroughs of separate chaining and open addressing, four ways to break a hash table, and reference Java for both strategies.

A graph is any set of things connected however you like — no root, no promise you won't loop. Adjacency matrix versus list, BFS and DFS as the same code with a swapped container, and shortest paths with and without weights. Every diagram runs; every one can be stepped a line at a time.

Four data structures built from the same piece of plumbing: a node that knows where the next node lives. Interactive walkthroughs of singly and doubly linked lists, stacks, and queues, with the four edge cases every implementation has to survive and reference Java code for all of them.

An interactive walkthrough of what a binary tree really is: the ADT contract, insert / search / height / print stepped through one line at a time, and a live thirty-node BST you can click to delete from. Ends on the three deletion cases and why case three is the only genuinely awkward one.