size()

/* Return the size of the BST (total number of nodes) */
public int size() {
    return size(root);                          // 1
}

/* Private size() method */
private int size(Node<Key, Value> x) {
    if (x == null) {                            // 2
        return 0;                               // 2
    }
    return x.size;                              // 3
}