Graph
Nodes + edges. The most versatile data structure — model social networks, road networks, dependencies, neural networks, anything with relationships.
Directed/undirected, weighted/unweighted, cyclic/acyclic (DAG). Representation: adjacency matrix (fast lookup, lots of memory), adjacency list (compact, sequential). Classic algorithms: BFS, DFS, Dijkstra (shortest path), A*, MST (Kruskal/Prim), Tarjan SCC. Graph databases (Neo4j, Memgraph, TigerGraph) specialize storage. NetworkX (Python) + igraph + Boost Graph are common libraries.