>>41
const buckets = a.reduce((buckets, current) => {
if (!buckets[current.kind]) {
buckets[current.kind] = [current];
} else {
buckets[current.kind].push(current);
}
return buckets;
}, {});
const b = Object.values(buckets)
.filter((items) => items.length === 1)
.flat();

速度は測ってないけどたぶんO(n^2)→O(n)になって速いはず