Data structures

Lessons in this group, roughly in build order:

• arrays-and-dynamic-arrays — contiguous storage, O(1) index, amortized append
• linked-lists — pointer-chained nodes, O(1) splicing, no random access
• stacks-and-queues — LIFO and FIFO access disciplines
• hash-tables — average O(1) key→value via hashing + collision handling
• trees — hierarchical order: BST, AVL/red-black, B-tree
• heaps-and-priority-queues — always-extract-the-extreme, array-backed heap
• graphs — vertices + edges; BFS/DFS and what they unlock
• tries — prefix trees for string keys