public class Pair<K, V> {
    private K key;
    private V value;

    public Pair(K key, V value) {
        this.key = key;
        this.value = value;
    }

    public K getKey() {
        return key;
    }

    public V getValue() {
        return value;
    }

    // 交换 key 和 value
    public static <K, V> Pair<V, K> swap(Pair<K, V> pair) {
        return new Pair<>(pair.getValue(), pair.getKey());
    }

    @Override
    public String toString() {
        return "Pair{key=" + key + ", value=" + value + "}";
    }
}