public class ST<Key extends Comparable<Key>, Value> {
    private TreeMap<Key, Value> st;

    private Function<Key, Value> func;

    public ST(Function<Key, Value> func) {
        this.st = new TreeMap<>();
        this.func = func;
    }

    public Value get(Key key) {
        if (key == null) {
            throw new IllegalArgumentException("calls get() with null key");
        }
        if (st.containsKey(key)) {
            return st.get(key);
        }
        Value value = func.apply(key);
        st.put(key, value);
        return value;
    }

    public Value delete(Key key) {
        if (key == null) {
            throw new IllegalArgumentException("calls delete() with null key");
        }
        return st.remove(key);
    }

    public int size() {
        return st.size();
    }

    public Iterable<Key> keys() {
        return st.keySet();
    }

}