import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Implementation of {@code Multimap} that uses an {@code ArrayList} to store the values for a given
 * key. A {@link HashMap} associates each key with an {@link ArrayList} of values.
 *
 * <p>When iterating through the collections supplied by this class, the ordering of values for a

 * keys according to the above key ordering, and for each key they iterate across the values
 * according to the value ordering.
 *
 * <p>The multimap does not store duplicate key-value pairs. Adding a new key-value pair equal to an
 * existing key-value pair has no effect.
 *
 * <p>Null keys and values are permitted (provided, of course, that the respective comparators

import java.util.HashMap;
import java.util.List;
import java.util.Map;
import org.checkerframework.checker.nullness.qual.Nullable;

/**
 * Implementation of {@code Multimap} that uses an {@code ArrayList} to store the values for a given
 * key. A {@link HashMap} associates each key with an {@link ArrayList} of values.
 *
 * <p>When iterating through the collections supplied by this class, the ordering of values for a