Challenge: DFS Topological Sort

Start with the idea

Solve the exact task, not just the visible example: Return reverse-postorder for ascending neighbors. On input n=4 edges=0->2,1->2,2->3, predict the output before opening the answer reveal. The revealed output is a test oracle for this case, while private checks exercise other legal cases. Start by stating the input grammar and output format in your own words. Parse the complete input, including surrounding whitespace, before relying on any convenient positions in the example.

Build the solution around one maintained claim: Append a node after all descendants; a gray-to-gray edge proves a directed cycle. Decide what state represents that claim, how it is initialized, what a single update does, and when the algorithm stops. A graph method needs an explicit vertex-and-edge representation. For traversal, define when a vertex becomes seen: on insertion into the frontier or on removal. Trace the visible input by hand and record at least two intermediate states, not only the final answer. If the trace cannot justify your predicted output, there is a gap between the idea and the implementation.

Use the Rust sketch as a guide, not a substitute for a complete solve function. The supplied Rust sketch is a starting point. Identify what it stores, how one iteration changes it, and whether it already solves the exact task or only demonstrates a related operation. An adjacency list and VecDeque make breadth-first work explicit. Mark before enqueuing to avoid duplicate frontier entries.