deleteMin()

/* Delete the node with the minimum key */
public void deleteMin() {
    root = deleteMin(root);                                 // 1
}

/* Private deleteMin() method */
private Node<Key, Value> deleteMin(Node<Key, Value> x) {
    if (x.left == null) {                                   // 2
        return x.right;                                     // 2
    }

    x.left = deleteMin(x.left);                             // 3
    x.size = 1 + size(x.left) + size(x.right);              // 4

    return x;                                               // 5
}