Topological Sort

You can't take Data Structures before Intro to Programming. That one prerequisite is a directed edge, and a surprising number of problems are just that edge multiplied: order the nodes so every one of them lands after the things it depends on. Topological sort hands you that order, or tells you it's impossible because a cycle means something eventually depends on itself. Course Schedule, build order, task scheduling, they're the same problem with the nouns swapped out. When a prompt says "prerequisites" or "dependencies," it's showing you a directed graph and asking for an order.