InvertIndexer.java

import java.util.*;

public class InvertIndexer {

    private Map<String, Set<String>> invertedIndex = new HashMap<>();

    public void index(String[][] inputSet) {
        for (String[] input : inputSet) {
            String word = input[0];
            Set<String> words;
            for (int i = 1; i < input.length; i++) {
                String tag = input[i];
                if ((words = invertedIndex.get(tag)) == null) {
                    words = new HashSet<>();
                }
                words.add(word);
                invertedIndex.put(tag, words);
            }
        }
        System.out.println(invertedIndex);
    }

    public Set<String> searchByTags(String[] input) {
        Set<String> allTags = new HashSet<>();
        for (String tag : input) {
            Set<String> tags = this.invertedIndex.get(tag);
            if (tags != null) {
                if (allTags.isEmpty()) {
                    allTags = tags;
                } else {
                    allTags.retainAll(tags);
                }
            }
        }
        return allTags;
    }

    public static void main(String[] args) {

        // create invert index
        // the first element is the word and the rest of elements are tags
        String[][] inputSet = {
                new String[]{"구글", "안드로이드", "검색", "모바일"},
                new String[]{"애플", "iOS", "모바일", "맥북"}
        };
        InvertIndexer indexer = new InvertIndexer();
        indexer.index(inputSet);

        // search by tags
        String testInputSet[][] = {
                new String[]{"iOS", "모바일"},
                new String[]{"모바일"},
                new String[]{"Windows"}
        };
        for (String[] input : testInputSet) {
            System.out.println(Arrays.asList(input) + " => " + indexer.searchByTags(input));
        }

    }

}