Implement a Dependency Resolver (Topological Sort)

Problem Given a set of build layers (or packages) and their dependencies, return a valid build order in which every layer appears after all the layers it depends on. If the dependencies contain a cycle, no valid order exists — detect and report it.

Input / Output

• Input: a list of nodes and directed dependency edges (A->B means B must be built before A).
• Output: a topologically ordered list of nodes, or an error identifying a cycle.

Constraints

• Up to 10^4 nodes and their edges.
• A node with no dependencies can be built at any time; ordering among independent nodes is unconstrained.

Example

• deps A->B, B->C => build order C, B, A.
• deps A->B, B->A => error: cycle between A and B.