Split graph folder into graph and tree

graph/directed_acyclic_graph.h

@@ -0,0 +1,16 @@
+#ifndef DIRECTED_ACYCLIC_GRAPH_H_
+#define DIRECTED_ACYCLIC_GRAPH_H_
+
+#include "digraph.h"
+
+namespace graph {
+
+template<typename T>
+class DirectedAcyclicGraph {
+public:
+  DirectedAcyclicGraph() = default;
+};
+
+} // namespace graph
+
+#endif