u01: Null pointers in main after delete

u01/main.cpp

@@ -3,8 +3,10 @@
 int main(int argc, char **argv) {
   node *t = create_complete_tree(2, 4);
   delete t;
+  t = nullptr;
   node *root = new node("root");
   root->add_child(new node("left child"));
   root->add_child(new node("right child"));
   delete root;
+  root = nullptr;
 }