Kahn's algorithm for topological sort

Kahn_algorithm.cc

template<typename NodeRef>
class Graph {
public:
  std::vector<NodeRef> allNodes() const;
  std::vector<NodeRef> getChildren(NodeRef) const;
};

template<typename NodeRef>
std::vector<NodeRef> TopologicalSort(const Graph<NodeRef>& G) {
  std::map<NodeRef, unsigned> RefCount;
  for (NodeRef From : G.allNodes()) {
    RefCount.emplace(From, 0);
    for (NodeRef To : G.getChildren(From)) {
      RefCount[To]++;
    }
  }

  std::queue<NodeRef> Q;
  for (auto& Entry : RefCount) {
    if (Entry.second == 0) {
      Q.push(Entry.first);
    }
  }

  std::vector<NodeRef> Results;
  while (!Q.empty()) {
    NodeRef From = Q.front();
    Q.pop();
    Results.push_back(From);
    for (NodeRef To : G.getChildren(From)) {
      if (--RefCount[To] == 0) {
        Q.push(To);
      }
    }
  }
  return Results;
}