class List {
private:

  class Node {
  public:
    int data;
    Node * next;
    Node (int data, Node *next) {
      Node::data = data; Node::next = next;
    }
  };

  Node *head;

public:

  // default contstructor: creates empty list
  List () { head = NULL; };

  // destructor: destroys list
  ~List ();

  // outputs list contents to cout
  void outputList () const;

  // adds a value at the beginning of the list
  void insert (int value);

  // Method "sumEvenValues" returns the sum of the nodes in the list containing even values.
  // Note that "containing" refers to the *contents* of the node, not its position in the list.
  // If the list has no even values, it returns 0.
  int sumEvenValues() const;

  // Method "deleteEveryThirdNode" deletes the 3rd, 6th, 9th, etc. node of the list.
  // Note that the list starts with the first (not "zeroth") node.
  // If the list has fewer than three nodes, this method has no effect.
  void deleteEveryThirdNode();

};