(list)+map function

void return, just maps f onto every value in the linked list till
nullity.

list.cpp

@@ -54,6 +54,15 @@ List<T> *reverse(List<T> *lst, List<T> *prev = nullptr)
   return reverse(next, lst);
 }
 
+template <typename T, typename U>
+void map(List<T> *lst, U (*f)(T))
+{
+  if (!lst)
+    return;
+  lst->value = f(lst->value);
+  map(lst->next, f);
+}
+
 template <typename T>
 std::ostream& operator<<(std::ostream& ostream, const List<T> *lst)
 {