Arrays & Strings: Contiguous memory, O(1) indexing, dynamic arrays that double, 2D arrays and immutable strings.
Linked Lists: Nodes and pointers: singly, doubly and circular lists, and when they beat arrays.
Stacks & Queues: Last-in-first-out and first-in-first-out, deques, and how both are built on arrays.
Hash Tables: Hash functions, buckets, collisions and resizing: why lookups are O(1) on average.
Trees: Vocabulary, the types of tree, binary search trees and the four traversals.
Heaps & Priority Queues: A complete tree stored in an array: always know the minimum, in O(log n) per change.
Graphs: Nodes and edges: directed, weighted, cyclic; adjacency lists vs matrices; BFS and DFS.
Choosing a Data Structure: One table of every structure and operation, and a decision guide for picking one.
Reading Constraints: The problem's biggest hint: target complexity from n, int vs long, negatives, value ranges, memory, recursion depth and modulo 10^9 + 7.