Topological Sort:

• Orders nodes so that every prerequisite appears before the node that depends on it.
• 让每个前置节点都排在依赖它的节点前面。

Common Approaches

• BFS topological sort (Kahn’s Algorithm)
  • Use indegree to count how many prerequisites each node still has
  • Repeatedly process nodes with indegree == 0
  • Usually, no visited set is needed
• DFS
  • Explore as deep as possible first
  • After all neighbors are processed, add the current node to the result
  • This is based on postorder traversal
  • 先一路走到最深，等后面的都处理完，再把当前节点放进答案