LV 3

function solution(genres, plays) {
  let res = [];
  const musicObject = genres.reduce((genreObject, genre, index) => {
    genreObject[genre] = genreObject[genre]
      ? [
          genreObject[genre][0] + plays[index],
          genreObject[genre][1].concat([index]),
        ]
      : [plays[index], [index]];

    return genreObject;
  }, {});
  Object.entries(musicObject)
    .sort((a, b) => b[1][0] - a[1][0])
    .forEach(([genre, [totalPlays, indexArr]]) => {
      const sortedByPlays = indexArr.sort((a, b) => plays[b] - plays[a]);
      sortedByPlays.slice(0, 2).forEach((index) => res.push(index));
    });

  return res;
}