u01: Make destructor work with recursive trees

u01/node.cpp

@@ -16,8 +16,11 @@ node::node(const std::string &name) : name(name) {
 
 node::~node() {
   std::cout << "enter ~node() of \"" << get_name() << "\"" << std::endl;
+  this->destroy = true;
   for (node *child : children) {
-    delete child;
+    if (!child->destroy) {
+      delete child;
+    }
   }
   std::cout << "leave ~node() of \"" << get_name() << "\"" << std::endl;
 }

u01/node.h

@@ -18,6 +18,9 @@ public:
 private:
   std::string name;
   std::vector<node *> children;
+  bool destroy =
+      false; // gets set by the destructor,
+             // allows deletion of recursive trees with a recursive destructor
   static unsigned int node_id;
 };