Split graph folder into graph and tree

tree/tree.h

@@ -0,0 +1,19 @@
+#ifndef TREE_H_
+#define TREE_H_
+
+#include "graph/digraph.h"
+using namespace graph;
+
+namespace tree {
+
+template<typename T>
+class Tree : public Digraph<T> {
+public:
+  Tree() = default;
+
+  Tree(std::map<T, std::set<T>> G) : Digraph<T>::Digraph(G) {}
+};
+
+} // namespace tree
+
+#endif