Split graph folder into graph and tree

tree/in_tree.h

@@ -0,0 +1,20 @@
+#ifndef IN_TREE_H_
+#define IN_TREE_H_
+
+#include "directed_rooted_tree.h"
+using namespace graph;
+
+namespace tree {
+
+template<typename T>
+class InTree : public DirectedRootedTree<T> {
+public:
+  InTree() = default;
+
+  InTree(std::map<T, std::set<T>> G)
+    : DirectedRootedTree<T>::DirectedRootedTree(G) {}
+};
+
+} // namespace tree
+
+#endif