diff --git a/u01/node.h b/u01/node.h
index 1632899..6d15ef4 100644
--- a/u01/node.h
+++ b/u01/node.h
@@ -1,3 +1,5 @@
+#ifndef _NODE_H
+#define _NODE_H
 #include <memory>
 #include <string>
 #include <vector>
@@ -19,3 +21,4 @@ private:
 };
 
 node *create_complete_tree(std::size_t nr_child_nodes, unsigned int tree_depth);
+#endif