function quickSort(arr) {
  if (arr.length <= 1) return arr;
  const pivot = arr[0];
  const left = arr.filter((x, i) => i > 0 && x < pivot);
  const right = arr.filter(x => x >= pivot);
  return [...quickSort(left), pivot, ...quickSort(right)];
}

class Node {
  constructor(val) {
    this.val = val;
    this.next = null;
  }
}