Make class Graph abstract and make derived classes Digraph and SCC

graph/digraph.h

@@ -0,0 +1,26 @@
+#ifndef DIGRAPH_H_
+#define DIGRAPH_H_
+
+#include "graph.h"
+
+#include <algorithm>
+
+namespace graph {
+
+template<typename T>
+class Digraph : public Graph<T> {
+public:
+  Digraph() = default;
+
+  // Reverse graph directions
+  Digraph<T> reverse();
+};
+
+template<typename T>
+Digraph<T> Digraph<T>::reverse() {
+  return Digraph<T>();
+}
+
+} // namespace graph
+
+#endif