프로그래머스 level3 베스트앨범프로그래머스 level3 베스트앨범

import java.io.IOException;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
class Solution {
    static class Song implements Comparable<Song> {
		int id, play;
		String genre;

		Song(int id, int play, String genre) {
			this.id = id;
			this.play = play;
			this.genre = genre;
		}

		@Override
		public int compareTo(Song o) {
			if (this.play == o.play) {
				return this.id - o.id;
			} else {
				return -(this.play - o.play);
			}
		}
	}

    public int[] solution(String[] genres, int[] plays) {
        ArrayList<Song> songList = new ArrayList<>();
		;
		HashMap<String, Integer> total_genres_map = new HashMap<>();

		for (int i = 0; i < genres.length; i++) {
			songList.add(new Song(i, plays[i], genres[i]));
			if (total_genres_map.containsKey(genres[i]))
				total_genres_map.put(genres[i], total_genres_map.get(genres[i]) + plays[i]);
			else
				total_genres_map.put(genres[i], plays[i]);
		}

		Collections.sort(songList, new Comparator<Song>() {
			@Override
			public int compare(Song s1, Song s2) {
				if (s1.genre.equals(s2.genre)) {
					return s1.compareTo(s2);
				} else {
					return -(total_genres_map.get(s1.genre) - total_genres_map.get(s2.genre));
				}
			}
		});
		HashMap<String, Integer> albumMap = new HashMap<>();
		ArrayList<Integer> bestAlbum = new ArrayList<>();
		for (Song song : songList) {
			if(!albumMap.containsKey(song.genre))
			{
				albumMap.put(song.genre, 1);
				bestAlbum.add(song.id);
			}
			else if(albumMap.get(song.genre)==1)
			{
				albumMap.put(song.genre, 2);
				bestAlbum.add(song.id);
			}
			else if(albumMap.get(song.genre)==2)
				continue;
		}
		int[] answer = new int[bestAlbum.size()];
        for(int i = 0 ; i < answer.length ; ++i)
	        answer[i] = bestAlbum.get(i);
        return answer;
    }
}

Collections.sort(songList, new Comparator() {
@Override
public int compare(Song s1, Song s2) {
if (s1.genre.equals(s2.genre)) {
return s1.compareTo(s2);
} else {
return -(total_genres_map.get(s1.genre) - total_genres_map.get(s2.genre));
}
}
});

음수 또는 0이면 객체의 자리가 그대로 유지되며, 양수인 경우에는 두 객체의 자리가 변경된다.